fabrication of zinc oxide piezoelectric nanostructures: a ... · structure capable of converting...

UNIVERSITY OF PORTO

Fabrication of Zinc Oxide PiezoelectricNanostructures: A route towards

Nanogeneration

by

Filipe Falcao de Oliveira

A thesis submitted in partial fulfillment for thedegree of Master in Physics Engineering

in theFaculty of Sciences

Physics and Astronomy Department

4th November 2014

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[email protected]

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Declaration of Authorship

I, Filipe Falcao de Oliveira, declare that this thesis entitled, ‘Nanostructured Zinc Oxide

Piezoelectric Nanogenerators’ and the work presented in it are my own. I confirm that:

This work was done wholly or mainly while in candidature for a research degree

at this University.

Where any part of this thesis has previously been submitted for a degree or any

other qualification at this University or any other institution, this has been clearly

stated.

Where I have consulted the published work of others, this is always clearly attrib-

uted.

Where I have quoted from the work of others, the source is always given. With

the exception of such quotations, this thesis is entirely my own work.

I have acknowledged all main sources of help.

Where the thesis is based on work done by myself jointly with others, I have made

clear exactly what was done by others and what I have contributed myself.

Signed:

Date:

i

”You have to learn the rules of the game. And then you have to play better than anyone

else.”

Albert Einstein

UNIVERSIDADE DO PORTO

ResumoFaculdade de Ciencias

Departamento de Fısica e Astronomia

Mestrado em Engenharia Fısica

por Filipe Falcao de Oliveira

A sociedade atual tem se tornado cada vez mais livre para realizar tarefas enquanto

anda, em viagem ou longe de um local fixo de trabalho, por causa dos mais recen-

tes desenvolvimentos em tecnologia, como laptops, smartphones, tablets ou smart-

watches. Mas esta qualidade de vida mais independente e apenas temporaria por

causa do uso de baterias e do seu tempo de vida limitado. Este trabalho realca uma

nova tecnologia (nanogeradores) que e capaz de captar energia mecanica do meio

ambiente e converte-la em electricidade, pronto para alimentar os nossos disposit-

ivos electronicos diarios e sensores de monitorizacao com a possibilidade de com-

plementar o uso de baterias, aumentando a vida util dos dispositivos ou alimentar

permanentemente pequenos aparelhos eletronicos sem a necessidade armazenar en-

ergia. Para isso, recorre a um material piezoeletrico, ou seja, um cristal com estrutura

nao centrosimetrica capaz de converter deformacoes mecanicas, tais como torcao,

flexao ou compressao, em energia electrica. Tipicamente, um nanogerador e com-

posto por materiais nanoestruturados capazes de converter energia devido a flexib-

ilidade, miniaturizacao e maior taxa de conversao quando comparada com bulk. O

oxido de zinco (ZnO) e um material com estrutura cristalina wurtzite, possuindo as-

sim propriedades piezoelectricas. Neste trabalho conseguiu-se produzir e caracter-

izar diferentes nanoestruturas de ZnO (filmes finos, nanofios e nanopartıculas) atraves

de processos quımicos simples e de baixo custo e aplicar com exito os nanofios na

producao de um nanogenerator piezoelectrico. Com deformacoes preliminares nao

controladas foi possivel obter cerca de 50mV de tensao de saıda comprimindo o nano-

gerador. Alem disso, foi realizado um estudo atravs de metodos numericos para inferir

a influencia dos parametros geometricos de um nanofio piezoelectrico no potencial de

saıda. Os resultados obtidos neste trabalho abrem portas para o desenvolvimento e

optimizacao destes dispositivos, juntamente com a possibilidade de aplicacoes wear-

able num futuro proximo.



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UNIVERSITY OF PORTO

AbstractFaculty of Sciences

Physics and Astronomy Department

Master in Physics Engineering

by Filipe Falcao de Oliveira

The present society has become increasingly free to carry out their tasks while walk-

ing, traveling or away from a fixed workplace due to of the most recent developments in

technology, such as laptops, smartphones, tablets or smartwatches. But this increas-

ingly independent quality of life is only temporary because of the use of batteries and

their limited lifetime. This work sheds light over a novel technology (nanogenerator) that

is capable of harvesting unattended mechanical energy from the surrounding environ-

ment and convert it into electricity, ready to feed our daily electronic gadgets and mon-

itoring sensors with the possibility to complement the use of batteries, increasing the

working lifetime or permanently feed small electronics without requiring an energy stor-

age. To do so, it recurs to a piezoelectric material, i.e. a crystal with non-centrosymetric

structure capable of converting mechanical deformations, such as twisting, bending or

compression, into electrical energy. Typically, a nanogenerator is composed by energy

converting nanostructured materials because of the increased flexibility, miniaturization

and higher conversion rate when compared to bulk. The zinc oxide (ZnO) is a crystal

with wurtzite structure, thus having piezoelectric properties. In this work one was able

to produce and characterize different ZnO nanostructures (thin films, nanowires and

nanoparticles) and successfully apply the nanowires to produce a novel piezoelectric

nanogenerator based on low-cost and simple chemical processes. With preliminary un-

controlled deformations one was able to obtain approximately 50mV of output potential

upon compressing the nanogenerator. Also, a numerical method study was performed

to infer the influence of the geometric parameters of a piezoelectric nanowire in the out-

put potential. The results obtained in this work open doors for future development and

optimization of these devices along with the possibility for applications in daily wear-

ables.



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Acknowledgements

This project was developed in the Instituto de Fısica dos Materiais da Universidade do

Porto (IFIMUP and IN - Institute of Nanoscience and Nanotechnology) facilities, using

particularly the Laboratory of Self-Organized Nanostructures, x-ray diffraction equip-

ment and experimental laboratories, and the Centro de Materiais da Universidade do

Porto (CEMUP) Cleanroom facilities. For that I want to thank my IFIMUP-IN colleagues

Celia Sousa, Pedro Fernandes, Maria Susano and Kumaresa Vanji for all the help

they provided during the experimental procedures in the Laboratory of Self-Organized

Nanostructures.

I want to specially acknowledge Mariana Proenca, my co-supervisor, for all the labor-

atory teaching she provided and all the technical help in solution preparation and data

analysis, which were of great importance to this work. Also I want to thank her for all

the time spent in this project.

Furthermore, I want to thank Joao Ventura for all the inspiration, motivation and support

whenever the work reached a dead end. The first time I heard of this technology was

through him and so I want to thank him for this great project opportunity and all the

open possibilities that it can lead us to.

I want to thank Maria Paz for teaching me how to operate the x-ray diffraction equip-

ment and how to analyze the results. Also want to thank Francisco Carpinteiro for the

dedication and time spent in this project and for the developed measurement set-up.

Due to his dedication, understanding and great work, I want to thank Rui Rocha, the

scanning electron microscope operator, where his work was of major importance to

understand all the produced samples and the paths to follow to complete this project.

Finally, I want to leave a special thank to my colleagues, Bernardo Bordalo, Miguel

Rosmaninho and Sergio Naphulo for the company, feedback and discussion on each

others work, which helped me to improve my work and try different and better paths.

v

Contents

Declaration of Authorship i

Resumo iii

Abstract iv

Acknowledgements v

List of Figures ix

List of Tables xi

Scientific Work xii

Entrepreneurship Relations xiii

Abbreviations xiv

Physical Constants xv

Symbols xvi

1 Introduction 11.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.2 Energy Harvesting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.3 Nanogenerators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.3.1 Piezoelectric Nanogenerators . . . . . . . . . . . . . . . . . . . . . 51.4 Piezoelectric Zinc Oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.4.1 Introduction to Piezoelectricity . . . . . . . . . . . . . . . . . . . . 61.4.1.1 Piezoelectricity in one-dimension . . . . . . . . . . . . . . 81.4.1.2 Tensor Notation . . . . . . . . . . . . . . . . . . . . . . . 10

1.4.2 Zinc Oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111.5 Thesis Organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

vi

Contents vii

2 Experimental Techniques 152.1 Fabrication Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.1.1 Eletrochemical Depositon . . . . . . . . . . . . . . . . . . . . . . . 152.1.2 Spin-Coating Depositon . . . . . . . . . . . . . . . . . . . . . . . . 162.1.3 Hydrothermal Growth Process . . . . . . . . . . . . . . . . . . . . 172.1.4 Solochemical Method . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.2 Characterization Techniques . . . . . . . . . . . . . . . . . . . . . . . . . 192.2.1 Scanning Electron Microscopy . . . . . . . . . . . . . . . . . . . . 192.2.2 X-Ray Diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3 Fabrication of ZnO Nanostructures 213.1 Electrochemical deposition of ZnO thin films . . . . . . . . . . . . . . . . . 21

3.1.1 Experimental procedure . . . . . . . . . . . . . . . . . . . . . . . . 223.1.2 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . 23

3.1.2.1 Deposition at room temperature . . . . . . . . . . . . . . 233.1.2.2 Deposition at 80 C . . . . . . . . . . . . . . . . . . . . . 243.1.2.3 Deposition Current Transients . . . . . . . . . . . . . . . 263.1.2.4 Deposited Charge . . . . . . . . . . . . . . . . . . . . . . 28

3.1.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313.2 ZnO thin film obtained by a sol-gel chemical process . . . . . . . . . . . . 31

3.2.1 Experimental procedure . . . . . . . . . . . . . . . . . . . . . . . . 313.2.2 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . 323.2.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

3.3 Growth of ZnO nanowires though hydrothermal process . . . . . . . . . . 343.3.1 Experimental procedure . . . . . . . . . . . . . . . . . . . . . . . . 343.3.2 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . 35

1-Step . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352-Step . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

3.3.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403.4 Fabrication of ZnO nanoparticles by a solochemical method . . . . . . . . 40

3.4.1 Experimental procedure . . . . . . . . . . . . . . . . . . . . . . . . 403.4.2 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . 413.4.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

3.5 Chapter Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

4 Finite Element Methods Study 444.1 Hexagonal ZnO Nanowire . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

4.1.1 Numerical Methods . . . . . . . . . . . . . . . . . . . . . . . . . . 444.1.1.1 Mesh details and limiting information . . . . . . . . . . . 46

4.1.2 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . 464.1.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

5 Piezoelectric Nanogenerator 515.1 Composition and production . . . . . . . . . . . . . . . . . . . . . . . . . . 515.2 Automatized set-up for mesurement of piezoelectric output potential . . . 53

5.2.1 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . 535.3 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

Contents viii

6 Conclusions and Future Work 56

Bibliography 58

List of Figures

1.1 Schematic representation of energy harvesting through different proper-ties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2 Working principles of nanogenerators. . . . . . . . . . . . . . . . . . . . . 31.3 Different piezoelectric nanogenerators. . . . . . . . . . . . . . . . . . . . . 51.4 Illustrative representation of dielectric materials. . . . . . . . . . . . . . . 71.5 Schematic atoms distribution upon deformation. . . . . . . . . . . . . . . 81.6 Unity cell wurtzite crystalline structure. . . . . . . . . . . . . . . . . . . . . 12

2.1 Electrodeposition set-up for thin film deposition. Close-up image of three-electrode set-up. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.2 Spin-coating deposition process. . . . . . . . . . . . . . . . . . . . . . . . 172.3 Laurell WS-650S spinner. Precision hot plate for soft-baking. . . . . . . . 172.4 Representation of hydrothermal apparatus for ZnO NW growth on flat

substrates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.5 X-ray beams interacting with crystal’s periodic lattice. . . . . . . . . . . . . 19

3.1 Schematic representation of the electrochemical deposition experimentalset-up. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.2 SEM analysis of ZnO electrodeposition at room temperature for 0.1 M ofZNH. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.3 SEM analysis of ZnO electrodeposition at room temperature for 0.01 Mof ZNH. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3.4 SEM analysis of ZnO electrodeposition at 80 C for 0.1 M of ZNH. . . . . . 253.5 SEM analysis of ZnO electrodeposition at 80 C for 0.01 M of ZNH. . . . . 263.6 XRD spectra of typical electrodeposited ZnO thin film . . . . . . . . . . . 263.7 Deposition current transients during the electrodeposition process of se-

lected ZnO thin films. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273.8 Time dependence of the electrodeposition charge density of represent-

ative electrodeposited ZnO thin films. . . . . . . . . . . . . . . . . . . . . . 293.9 Estimated thickness of the electrodeposited thin films. . . . . . . . . . . . 303.10 Schematic representation of reflux process for ZnO sol-gel synthesis. . . 323.11 Morphological analysis of the spin-coated ZnO sol-gel thin film. . . . . . . 333.12 XRD spectra of the spin-coated thin film produced by a ZnO sol-gel and

used standard PET substrate. . . . . . . . . . . . . . . . . . . . . . . . . . 343.13 SEM images of 1-step hydrothermal samples for 25 mM concentration. . 363.14 SEM images of hydrothermal samples with 1 step for 50 mM concentration. 373.15 SEM images of hydrothermal samples as produced using a 2-step pro-

cess for 25 mM concentration. . . . . . . . . . . . . . . . . . . . . . . . . 38

ix

List of Figures x

3.16 SEM images of hydrothermal samples as produced using a 2-step pro-cess for 50 mM concentration. . . . . . . . . . . . . . . . . . . . . . . . . 38

3.17 XRD spectra of hydrothermally grown ZnO nanowires. . . . . . . . . . . . 393.18 Preparation steps of ZnO nanoparticles by solochemical method. . . . . . 413.19 SEM images of zinc oxide nanoparticles without an annealing step. . . . . 413.20 XRD spectra for the resulting ZnO nanopowder. Linear fit using Williamson-

Hall correlation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

4.1 Mesh definition for the nanowire geometry. . . . . . . . . . . . . . . . . . 464.2 Considered geometry of a zinc oxide nanowire. Electric potential as func-

tion of applied force for both parallel and perpendicular directions. . . . . 474.3 Electric potential as function of the height with W = 100 nm. . . . . . . . . 484.4 Output voltage obtained by varying the width with H = 3µm. . . . . . . . . 494.5 Resulting piezoelectric potential upon a constant force, varying heigth

and width. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

5.1 (a) Graphical representation of a complete piezoelectric nanogenerator.(b) Sectioned nanogenerator displaying involved components. . . . . . . . 52

5.2 Steps of production of a piezoelectric nanogenerator. . . . . . . . . . . . . 535.3 Metal box acting as Faraday cage, shielding the device from E.M. noise

with NG connected with cooper wires. Output potential obtained from thefirst functional piezoelectric nanogenerator. . . . . . . . . . . . . . . . . . 54

5.4 Deformation output potential mesurement set-up. . . . . . . . . . . . . . . 54

List of Tables

1.1 Coupling coefficient for different piezoelectric materials . . . . . . . . . . . 91.2 Abbreviated notation for tensors. . . . . . . . . . . . . . . . . . . . . . . . 111.3 ZnO piezoelectric constants . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.1 Parameters tuned during the electrochemical deposition process. . . . . . 223.2 Parameters tuned during the hydrothermal growth method: time (2.5 and

5 h); number of steps (1 or 2); concentration of zinc nitrate (25 and 50mM). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

3.3 Geometric parameters of hydrothermally grown ZnO NWs for differentsolution concentrations and process durations (1-step process). . . . . . 35

3.4 Geometric parameters of hydrothermally grown ZnO NWs for differentsolution concentrations and process durations (2-step process). . . . . . 37

xi

Scientific Work

Publications

F. F. Oliveira, M. P. Proenca, J. P. Araujo, J. Ventura, Output potential of ZnO nanowires:

Influence of geometrical parameters, Journal of Nanoscience and Nanotechnology (2014)

(Submitted)

Oral Communications

F. F. Oliveira, M. P. Proenca, J. P. Araujo, J. Ventura, Hexagonal ZnO nanowire based

piezoelectric nanogenerator at VIII Jornadas IFIMUP-IN, Porto, September 12, 2014

F. F. Oliveira, M. P. Proenca, J. P. Araujo, J. Ventura, Piezoelectric Nanogenerators: Feeding

the Future at IJUP’14 - 7 Encontro de Jovens Investigadores da Universidade do Porto,

Porto, February 12-14, 2014

Poster Presentations

F. F. Oliveira, M. P. Proenca, J. P. Araujo, J. Ventura, Fabrication of piezoelectric ZnO

nanowires for energy harvesting applications at 5th International Conference on Advanced

Nanomaterials, Aveiro, July 2-4, 2014

xii

Entrepreneurship Relations

Since the early stages of this project and along with the Master project of my colleague

Miguel Rosmaninho on thermoelectric nanogenerators, all the intervening persons,

Joao Ventura, Andre Pereira, Mariana Proenca, Miguel Rosmaninho and Filipe Falcao,

saw a great industrial potential in these novel technologies. So, when the opportunity

appear, we entered the iUP25K Business idea contest, promoted by Universidade do

Porto Inovacao (UPIN) and Parque de Ciencia e Tecnologia da Universidade do Porto

(UPTEC). Throughout the contest we developed a business model accordingly to our

ideas that consists in providing consulting, designing and prototyping of custom-made

nanogenerators for specific applications. Our target markets are the investing indus-

tries of textile and shoes. With this project we were finalists of the iUP25K contest.

This experience allowed us to apply for an entrepreneurship passport from Agencia

para a Competitividade e Inovacao, I.P. (IAPMEI) that promotes the development of

business ideas along for an year. At the same time we entered an Latino-Iberian con-

test, IDEUP from RedEmprendia and are already placed in the final stage that will take

place in Mexico City.

xiii

Abbreviations

AFM Atomic Force Microscopy

HMTA Hexamethylenetetramine

ITO Indium Tin Oxide

MEA Monoethanolamine

NG Nanogenerator

NT Nanotube

NW Nanowire

PET Polyethylene terephthalate

PMMA Poly(methyl mmethacrylate)

SEM Scanning Electron Microscopy

XRD X-Ray Diffraction

ZAD Zinc Acetate Dihydrate

ZNH Zinc Nitrate Hexahydrate

xiv

Physical Constants

Permittivity of Free Space ϵ0 = 8.854× 10−12 F.m−1

Faraday Constant F = 96485 C mol−1

xv

Symbols

ε strain dimensionless

S elastic compliance Pa−1

σ stress Nm−2

c Young’s modulus Pa

E electric field Vm−1

P electric polarization Cm−2

ϵ0 permittivity of free space Fm−1

χe electric susceptibility dimensionless

p dipole moment Cm

d piezoelectric strain coefficient CN−1

e piezoelectric stress coefficient Cm−2

k electromechanical coupling coefficient dimensionless

H nanowire height µm

W nanowire width nm

L bent nanowire arc length µm

θ nanowire bending angle

xvi

Para os meus pais, para o meu irmao e para a Marina.

xvii

Chapter 1

Introduction

Independence has always been a word of great meaning. Throughout the history of

mankind, the individual has searched for automatic and independent processes, which

can ease human labor. So far, the introduction of machines has widely fulfilled this

purpose, realizing almost every single task automatically. Yet, all systems still depend

on one thing: an external power source, without which their automatic work runs out.

During the last two decades great advances have been made on nanofabrication and

nanotechnology, leading the production of integrated electronics, microprocessors, micro-

devices and micro-systems to a worldwide practice and their presence in a large range

of equipment and technology. While functional systems and corresponding compon-

ents become smaller due to research and development, the electric sources required

for their proper functioning are still based on temporary storages such as batteries and

capacitors. To overcome this difficulty, one needs to develop nanoscale components

capable of electrical energy generation through a conversion process. This is where

nanogenerators (NGs) come into scene [1, 2, 3]. In a world where integrated circuits

became top of technology, used daily in our smartphones, tablets, personal computers,

calculators, security systems, sensors or transportations, the dream of self-powered

devices directly generating electricity from the environment is finally within our grasp.

Through various environmental conditions such as contact pressure, twisting or bend-

ing, fluid oscillation, friction or heating, a nanogenerator can produce electricity con-

verting a fraction of this unattended energies into electrical power.

Amongst others, the application of nanogenerators in consumer electronics will lead to

an increase of their autonomy and a great industrial opportunity, taking advantage of

their flexibility and small size. Also, the use of nanogenerators on small remote sensors

like environmental monitoring equipment and weather control sensors allows the per-

manent and auto-sufficient functioning of such systems without periodic maintenance

1

Introduction 2

and replacement. Furthermore, the application of nanogenerators in live beings (an-

imals or humans), opens new possibilities in medical treatments and diagnosis tech-

niques, such as the implantation (inside the human body) of self-powered biosensors

with wireless transmitters and a nanogenerator that will send biometric data while con-

verting (e.g.) mechanical to electrical energy [4].

1.1 Motivation

The nanogeneration technology is a recent development and it holds great potential in

future applications, thus being a great topic of research, exploring the possibilities of

producing low-cost nanogenerators with optimized potential for daily applications. Dr.

Zhong Lin Wang, a functional materials researcher, is the pioneer on nanogeneration

technology and nanogenerator development [5, 6]. The piezoelectric nanogenerator is

one of the most promising devices as it has the ability of harvesting energy from a wide

range of sources and under various conditions. Therefore, motivated by the previous

works of Dr. Wang’s group and taking advantage of the cutting edge techniques of

the Physics and Astronomy Department and the IFIMUP-IN infrastructures, the main

objective of this thesis was to develop a piezoelectric nanogenerator based on ZnO

nanostructures [7, 8].

1.2 Energy Harvesting

Electricity

Thermoelectric

Triboelectric

Photovoltaic

Piezoelectric

Surroundingenvironment

FIGURE 1.1: Schematic representation of energy harvesting through different proper-ties.

Energy harvesting is defined as the ability to collect electrical energy from the sur-

rounding environment, i.e. the capacity to convert unattended and unexplored sources

Introduction 3

of energy. Through different properties and technologies, it is possible to convert mech-

anical and thermal energy into useful electricity. This sources can be the wasted heat

from smelting process or industrial burnings, the heating from the incident sun rays, the

movement wind flow and water streams present in nature, or in a smaller scale, the

daily movement of a human being, the movement of its clothes and shoes and even the

body heat and motion.

1.3 Nanogenerators

The natural environment provides us with several different sources of unexplored en-

ergy, such as wind oscillation, water streams, sun heating and living beings’ movement.

But it also presents us with different materials capable of converting such natural energy

into useful electricity, such as thermoelectric, triboelectric and piezoelectric materials

(Fig. 1.1). Thermoelectric materials, like lead telluride (PbTe), bismuth telluride (Bi2Te3)

or cadmium telluride (CdTe), allow the production of electricity from a thermal gradient,

or the control of temperature through an electric current [2, 9, 10, 11]. Triboelectric ma-

terials possess the property of becoming electrically charged upon friction, contact or

adhesion with other triboelectric material, creating positive or negative charges depend-

ing on the material’s tendency to gain or lose electrons. Materials with positive charge

tendency, such as nylon, glass, silk, aluminium, poly(methyl methacrylate) (PMMA) or

quartz, lose electrons when coming into contact with materials of opposite tendency

like amber, resins, polystyrene, silicon, Kapton or teflon, which gain electrons in the

process [3, 12, 13]. Piezoelectric materials, such as barium titanate (BaTiO3), lead ti-

tanate (PbTiO3), lead zirconium titanate (PZT; PbZrxTi1-xO3) or zinc oxide (ZnO), allow

the conversion of mechanical energy, in forms of oscillation, vibration, contact pressure

or bending, into electricity [14].

FIGURE 1.2: Different working principles of nanogenerators: (a) thermoelectric [2], (b)triboelectric [15] and (c) piezoelectric [14] nanogenerators.

A nanogenerator is composed of nanostrutured materials possessing the above prop-

erties, thus converting energy from different sources into electrical energy. Thermo-

electric nanogenerators [Fig. 1.2(a)] rely on the Seebeck effect, in which an electric

Introduction 4

current is generated between two different metals at different temperatures. When a

temperature difference is created, the mobile charge carriers in the material migrate

to the cold side and leave behind the immobile nuclei at the hot side, giving rise to a

thermoelectric voltage. The voltage measured between the two ends of the junction

is proportional to the corresponding temperature difference with a proportionality con-

stant known as Seebeck coefficient, or thermoelectric power [2, 16, 17, 18]. Although

being able to convert wasted heat in various situations, thermoelectric nanogenerators

present major downsides when miniaturized, including the difficulty to maintain a high

temperature difference between the two ends.

The triboelectric effect is a contact or slide effect in which two materials become elec-

trically charged after touching due to its tendency of charge transfer. Such effect is

typically regarded as prejudicial because it can lead to ignition, damage on electronics

and dust attraction. However Wang et al. have recently demonstrated nanogenerat-

ors capable of taking advantage of triboelectricity by turning it into effective electricity

[3, 15, 19, 20, 21, 22]. This device, as shown in Fig. 1.2(b), is composed of two ma-

terials with different polarity and triboelectric property, a spacer between them and two

electrodes. Although they seem efficient and well prepared for various configurations,

their working conditions (including the necessary operating environment) are restricted,

so that applications are specific and in proper measure. Also, the necessary configur-

ations are limited by the contact or friction between two different materials.

The piezoelectric effect is a material property that allows the creation of electric poten-

tial by means of deformation of the material, or vice-versa. Applying a tensile or com-

pressive force to a non-centrosymmetric crystalline material leads to the displacement

of the positive and negative centers of charge, inducing a piezoelectric polarization

and opposite charged surfaces in the material. Electrical energy is then obtained from

the piezoelectric material and injected in an external circuit directly from two contacts.

The same principle is applied to piezoelectric nanogenerators, as seen in Fig. 1.2(c).

Wang et al. first developed a piezoelectric nanogenerator composed of ZnO nanowires

(NWs) which produced electrical energy by the deformation induced by a platinum tip

[1]. Different configurations and more efficient NGs were then developed, increasing the

conversion efficiency and the application possibilities [23, 24, 25, 26]. The applications

for piezoelectric NGs are almost infinite because of the possibility to use all flexible and

dynamic surfaces like cloth or shoes, touchable electronics or even the human body, to

produce electricity.

Introduction 5

1.3.1 Piezoelectric Nanogenerators

Because of the simplicity in the working mechanism of piezoelectric NGs, it is possible

to use them in almost every environment, taking advantage of different phenomena, like

the wind passing through a tree to feed a fire detection sensor, or the water currents in

a river to feed micro-systems of water analysis. It is also possible to use deformations

caused by passing people in security systems and feed a detection mechanism. Finally,

one can even imagine a biosensor placed inside the human body permanently fed by

the pressure of the blood flux or muscle strain generated energy.

FIGURE 1.3: Different piezoelectric NGs: (a) first piezoelectric NG with an atomicforce microscopy Pt tip [1]; (b) a multi-layered integrated NG [23]; (c) piezoelectricVING configuration [14] and (d) an integrated piezoelectric NG on flexible substrate

[25].

The first reported piezoelectric NG device [1] [Fig. 1.3(a)] was composed of an array of

ZnO NWs and an atomic force microscope with a platinum tip was used to deform the

nanowires, creating a piezoelectric potential. In this device, a Schottky barrier, i.e. a

metal-semiconductor interface that acts as a diode to the electric current, was created

at the point of contact between the Pt tip and the ZnO nanowires, blocking the charge

flow until a certain point and then releasing all the accumulated charge. A maximum

output voltage of 6.5mV was achieved. This configuration presented crucial limitations

such as low output power density (around 1mWcm−2 for a resonance frequency of

10MHz), single deformation possibility and reduced applicability, leading to a search

for novel and more efficient configurations. The same group developed a piezoelectric

nanogenerator using the same concept, but in which the deformation of the NWs was

induced by an array of metallic tips grown on one side of a substrate, while the other

side was filled with ZnO NWs, allowing the stacking of several substrates. This con-

figuration could be deformed by contact forces or pressure variations, i.e., vibrational

Introduction 6

waves colliding against the device. For a four-layer integrated NG [Fig. 1.3(b)], an out-

put power density of 0.11 µWcm−2 at 62mV was obtained [23].

Integrated piezoelectric NGs were also reported, namely a vertical (VING) and a lateral

(LING) nanowire array nanogenerator. Both configurations present a complete pack-

aging, and upon a low bending strain of 0.19% the LING configuration resulted in a

peak output voltage of 1.26V. A series of three-layer stack VING [Fig. 1.3(c)] produced

an output potential of 243mV and an output power density peak of 2.7mWcm−3, ex-

tending the application possibilities of piezoelectric nanogenerators [14]. In this case,

the Schottky barrier is created by the direct contact of a platinum film on top of the

NWs. More recently, a nanogenerator composed of aligned ZnO NWs grown on both

sides of a flexible polyester substrate was presented, as shown in Fig. 1.3(d) [25, 26].

The NWs were coated with PMMA and the ends covered with a Cr/Au to create the

Schottky barrier. The device was attached to the inner surface of a car tire, converting

the deformation during rotation into electrical energy. A voltage of 1.5V and a current

of 25 nA were obtained, capable of lighting a liquid crystal display (LCD) screen, giving

a maximum power density of 70 µWcm−3 at the volume filled with ZnO nanostructures.

On later tests, the same device archived a maximum output voltage of 20V with 6µA of

maximum output current, resulting in a power density of 0.2W cm−3.

More recently, a super-flexible piezoelectric nanogenerator was developed with a com-

bined thickness of ∼18 µm and attached to a flag where, for a wind speed of 5.5m s−1,

the maximum voltage and current outputs were 50mV and 200 nA. The device was

also tested by pixing it over an eye and blinking, obtaining about 0.2V and 2 nA [27].

Also, a piezoelectric nanogenerator was developed, using PZT, with a 3.5 cm x 3.5 cm

functional area on a PET substrate reaching up to 200V and 8µA [28].

1.4 Piezoelectric Zinc Oxide

The usefulness of a piezoelectric nanogenerator is strongly dependent on the nano-

structured piezoelectric material quality. In this section we approach the piezoelectric

theory, the zinc oxide piezoelectric properties and why it was chosen for this project.

1.4.1 Introduction to Piezoelectricity

In 1880, Pierre and Paul-Jacques Curie [29] discovered that an external force applied

to certain crystals could generate charges on the surface of the crystal approximately

proportional to the applied mechanical stress, and that the reverse effect was also veri-

fied (deformation of a crystal due to an applied voltage). To this mechanical-electrical

Introduction 7

interaction was given the name of piezoelectricity, from the Greek term piezen which

means to press [30]. We can define the so-called direct piezoelectric effect as the abil-

ity to convert mechanical into electrical energy, or the reverse piezoelectric effect, in

which there is the conversion of electrical into mechanical energy (deformation of the

material).

Piezoelectricity has its origin in the arrangement of atoms in a crystal and is only

possible when the crystal presents a non-centrosymmetric structure. Based on stud-

ies of the crystal arrangements it is possible to catalog a piezoelectric material un-

der the broader category of dielectrics. In a total of 32 different crystal classes, 11

are centrosymmetric and therefore cannot be piezoelectric. The other 21 are non-

centrosymmetric, from which 20 are piezoelectric, as presented in Fig. 1.4.

FIGURE 1.4: Illustrative representation of dielectric materials, being piezoelectric asubgroup (adapted from [31]).

In a crystal structure one can define a center of charge, i.e., a position where all posit-

ively and negatively charged atoms are balanced, resulting in a neutrally charged crys-

tal. For a centrosymmetric crystal, the application of stress does not change the pos-

ition of the center of charge, so that the crystal remains neutrally charged. The same

does not apply for non-centrosymmetric crystals, for which a deformation moves the

center of charge, separating the center of positive charge from that of negative charge,

resulting in a non-neutral charged crystal. This separation leads to the formation of di-

poles, with the corresponding dipole moment pointing towards the more negative pole.

A electric field then appear that carry the potential based on the distance between

the more negatively charged position [anion (-)] and the more positively charged po-

sition [cation (+)]. The electric field is directly proportional to the dipole moment, so

that changing the dipole moment in the crystal will change the intensity of the field, as

represented in Fig. 1.5. It should be noted that the potential difference in piezoelec-

tric materials is a function of the variation of stress and thus such property is usually

applied to dynamic situations.

Introduction 8

FIGURE 1.5: Schematics demonstrating the arrangement of atoms and resulting dis-locations upon stress in a piezoelectric material (adapted from [31]).

1.4.1.1 Piezoelectricity in one-dimension

When an external stress is applied to a crystal, deformations or strains will appear.

In fact, and although stress does not cause strain, nor strain causes stress, they are

coupled to one another by Hooke’s Law [32],

ε = Sσ, (1.1)

where ε is the strain, S is the elastic compliance coefficient (Pa−1) and σ is the stress

(Nm−2). Young’s modulus is the inverse of the elastic compliance coefficient, so that

Eq.(1.1) can be written as:

σ = cε, (1.2)

where c is the elastic stiffness constant or Young’s modulus (c = 1/S).

On the other hand, when a voltage is applied across a piezoelectric crystal, an electric

field E (Vm−1) is created, which lines up the dipoles, resulting in a polarization P

(Cm−2) of the material, as follows:

P = ϵ0χeE, (1.3)

where ϵ0 is the permittivity of free space (ϵ0 = 8.854.10−12 [Fm−1]) and χe the electric

susceptibility (dimensionless). The polarization of a material is defined as P = N.p,

where N is the density of dipoles (m−3) and p is the dipole moment (Cm). In the

Introduction 9

Material Coupling coefficient (k )Quartz 0.1ZnO 0.33PZT 0.69

TABLE 1.1: Coupling coefficient for different piezoelectric materials [33].

presence of an electric field, an electric displacement D (Cm−2) also occurs, which is

given by:

D = ϵE = ϵ0E + P (1.4)

where ϵ is the permittivity of the material.

As the Curie brothers have demonstrated [29], the mechanical properties [Eq. (1.1)

and (1.2)] and electrical properties [Eq. (1.3) and (1.4)] can be related by:

Ppz = dσ, (1.5)

where d is the piezoelectric strain coefficient (CN−1), and pz symbolizes the piezo-

electric origin for the polarization P . Equation (1.5) represents the direct piezoelectric

effect. The reverse piezoelectric effect is given by:

εpz = dE. (1.6)

Both effects can also be formulated by applying Eq. (1.1) and (1.2) as

Ppz = dσ = dcε = eε

and

σpz = cεpz = cdE = eE

where e = d.c is the piezoelectric stress constant (Cm−2).

Finally, another important parameter to the piezoelectric phenomenon, presented in

Table 1.1, is the electromechanical coupling coefficient k. It indicates the amount of

mechanical energy converted to electricity (or vice-versa). This coupling coefficient is

defined as:

Introduction 10

k2 =k2e

1 + k2e, (1.7)

where

k2e =e233

c33ϵ33, (1.8)

where e is the piezoelectric stress constant, c is the Young’s modulus and ϵ is the

permittivity of the material with the indices 33 indicating an electric field parallel to the

z-axis and a resulting strain in the z-direction.

1.4.1.2 Tensor Notation

Although it is important to know how the piezoelectric effect behaves in a 1D situation,

for a realistic 3D situation we require a more complex model. When applying a stress,

lets say in the x-direction, strain will result, not only in the direction of the applied stress

but also in the perpendicular directions, y - and z-directions. In this situation, a three-

dimensional model is used based on tensors where the mechanical properties [Eq.

(1.1) and (1.2)] are described as:

εij = Sijklσkl (1.9)

and

σij = cijklεkl, (1.10)

and the electrical properties [Eq. (1.3) and (1.4)] become:

Di = ϵijEj . (1.11)

In these tensor equations, the indices i and j are the strain terms, where i, j = 1, 2, 3.

In the same way, k and l are the stress terms, where k, l = 1, 2, 3. In a 3D geometry,

Hooke’s law generates 81 possible terms of S or c although, due to symmetry (such

as εij = εji and σkl = σlk), one can reduce them to 36. Of the remaining terms, we

can use the Voigt’s notation [34], where the indices i and j can be abbreviated to λ

(ij → λ) whenever interchangeable. Table 1.2 shows the corresponding abbreviation,

which also stands for the k and l indices (kl→ µ).

Using the abbreviated tensor notation, Eq. (1.9) and (1.10) become:

Introduction 11

ij 11 22 33 23 32 31 13 12 21λ 1 2 3 4 5 6

TABLE 1.2: Abbreviated notation for tensors.

ελ = Sλµσµ (1.12)

and

σλ = cλµεµ, (1.13)

where µ, λ = 1, 2, 3, . . . 6.

Finally, the direct and reverse piezoelectric effects in a 3D geometry are given by:

Pi = diµσµ (1.14)

and

εµ = dtiµEi, (1.15)

with dt as the transpose of the piezoelectric strain tensor. In the case of this piezo-

electric coefficient, the index i indicates the direction of the field, with 1, 2 and 3 cor-

responding to the x, y and z directions, respectively. The second index, µ, refers to

the direction of the tensile (1, 2 and 3) or shear (4, 5 and 6) stress. A specific strain

piezoelectric coefficient value, e.g. d33 for an electric field parallel to the z-axis and a

resultant piezoelectric tensile strain in the z-direction, can then be estimated through

the amount of displacement of the material (∆l) and the values of the resulting voltage

V (d33 = ∆lV ).

1.4.2 Zinc Oxide

One of the defining characteristics of a piezoelectric material is the lack of center of

symmetry in the unit cell. Zinc oxide can present a wurtzite crystalline structure be-

longing to the P63mc space group, which results in a non-centrosymmetric structure,

as shown in Figure 1.6. ZnO also possesses a relatively high coupling coefficient, mak-

ing it excellent for a wide range of piezoelectric applications. The physical parameters

and constants associated with the ZnO piezoelectric crystal are given by the following

matrices [35]:

Introduction 12

FIGURE 1.6: Wurtzite crystal structure.

ϵij =

ϵ11 0 0

0 ϵ11 0

0 0 ϵ33

, (1.16)

diµ, eiµ =

0 0 0 0 d15 0

0 0 0 d15 0 0

d31 d31 d33 0 0 0

(1.17)

and

cλµ, Sλµ =

c11 c12 c13 0 0 0

c12 c11 c13 0 0 0

c13 c13 c33 0 0 0

0 0 0 c44 0 0

0 0 0 0 c44 0

0 0 0 0 0 c66

, (1.18)

where c66 = 12(c11 − c12) and S66 = 2(S11 − S22). All the piezoelectric constants and

parameter values are given in Table 1.3. Besides its piezoelectric properties, ZnO is

also a semiconductor material with a wide and direct bandgap of 3.3 - 3.4 eV which

is very important for high power and high temperature applications [36]. Additionally, it

can also emit UV photoluminescence when the material is excited, which is useful in

UV light applications such as photodetectors, light-emitting and laser diodes, that can

operate in the blue and ultraviolet spectrum region.

Introduction 13

ZnO can be deposited in thin films or grown in various nanostructures [8], such as

nanowires, nanotubes, nanorings or nanobelts, using a wide range of techniques like

electrochemical deposition (using a flat substrate or nanoporous templates) [37, 38],

chemical vapor deposition (CVD), magnetron sputtering, pulsed laser deposition (PLD),

ion beam deposition (IBD), thermal evaporation [7], spin-coated sol-gel [39, 40, 41, 42],

vapor liquid solid (VLS) [1, 43], solochemical process [44, 45] or hydrothermal growth

[14, 46, 47, 48].

Constant Value Unitϵ11/ϵ0 8.5 dimensionlessϵ33/ϵ0 10.9d33 12.4 10−12 CN−1 (10−12 mV−1)d31 -5.0d15 -8.3e33 1.57 Cm−2

e31 -0.36e15 -0.36c11 210 109 Nm−2

c12 121c13 105c33 211c44 43

TABLE 1.3: ZnO piezoelectric constants [35].

1.5 Thesis Organization

In this first chapter, we introduced the concept of nanogenerators, their most usual

types and working principles. Then, we described in detail the piezoelectric nanogen-

erator and the evolution of the device until now. Subsequently, we briefly introduced

the theory of piezoelectricity and the properties of the chosen material, ZnO, applied

to this effect. In chapter 2 we will describe the techniques used for the fabrication

(electrochemical deposition, spin-coating of sol-gel solution, hydrothermal process and

solochemical method) and characterization (scanning electron microscopy and X-ray

diffraction) of the ZnO nanostructures. In chapter 3 we will present the obtained nano-

structures, such as thin films (2D), nanowires (1D) and nanoparticles (0D) and sub-

sequent analysis. Afterwards, in chapter 4, we detail a numerical study where we used

a finite element method (FEM) to calculate the output piezoelectric potential varying

the geometric parameters of a ZnO hexagonal nanowire. Finally, chapter 5 shows the

Introduction 14

prototyped piezoelectric nanogenerator with the detailed step-by-step fabrication pro-

cess and deformation tests. Chapter 6 presents the final remarks regarding the project

herein described and future work.

Chapter 2

Experimental Techniques

2.1 Fabrication Techniques

In this section we describe the used experimental techniques to fabricate the ZnO nano-

structures with images and schematic representations of the set-ups. The subsec-

tions 2.1.1, 2.1.2, 2.1.3 and 2.1.4 explore the electrochemical deposition, spin-coating

depositon, hydrothermal growth process and solochemical method, respectively.

2.1.1 Eletrochemical Depositon

The electrochemical deposition method consists in the growth of a metallic or semi-

conductor material on the surface of conducting substrates, by the electrochemical

reduction of the respective ions present in an electrolyte solution. The electrolyte is

composed by chemical elements of the material of interest dissolved into a suitable

solvent. This process typically uses three electrodes: the substrate where the material

is grown acts as cathode or working electrode, a Pt mesh acts as anode or counter

electrode and a third electrode is used as a reference [49]. An electric current is ap-

plied between the counter and the working electrodes and precisely monitored with the

reference electrode. Typically there are three different modes of applied current: direct

current (DC), alternate current (AC) and pulsed electrodeposition (PED) which is an

alternation between the previous two modes. Figure 2.1 shows the experimental set-up

used in this work composed by laboratory material, homemade supports and a Sentek

R2 (Ag/AgCl) Double Junction reference electrode.

The electrodeposition of zinc oxide in thin films has been reported by Wellings et al.

using an aqueous solution of zinc nitrate at 80 C and using glass substrates with doped

15

Experimental Techniques 16

FIGURE 2.1: (a) Electrodeposition set-up for thin films deposition. (b) Close-up imageof three-electrode montage (from left to right: counter electrode, reference electrode

and working electrode).

fluorine tin oxide coatings. The potentials applied were−0.900 and−1.025V vs Ag/AgCl

in direct current, followed by an annealing step. This process resulted in polycrystalline

ZnO thin films of 0.4µm in thickness and exhibiting a crystallographic structure with a

(002) preferential orientation [37].

2.1.2 Spin-Coating Depositon

Sol-gel chemistry was developed in the 1960s to improve the low-temperature routes

for glass synthesis. A sol is defined as a colloidal suspension of solid particles in a

liquid, where a colloid is a suspension with a very small (1 to 1000 nm) dispersed phase

resulting in negligible gravitational forces. Existing interactions are dominated by short-

range forces, such as Van der Waals attraction and surface charges [50]. The sol-gel

chemistry allows the fabrication of thin films, nanotubes and nanowires with a reason-

ably high control in the growth rate and low cost. Metal oxides, especially silicon and

titanium oxides are the typical sol-gel deposited materials, normally applied to protect-

ive coating, thin films [51], fibers [52] and opto-mechanical devices [53]. The sol-gel

process starts with a percursor compound dissolved in a homogeneous solution which

undergoes a succession of transformations: (a) hydrolysis of the molecular precursor;

(b) polymerization via successive bimolecular additions of ions, forming oxo-, hydroxyl,

or aqua- bridges; (c) condensation by dehydration; (d) nucleation; and (e) growth. For

zinc oxide, several percursors have been used: nitrate, chloride, perchlorate, acet-

ylacetonate and alkoxides such as ethoxide and propoxide, being the acetate dihydrate

the most used [54].


FIGURE 2.2: Spin-coating deposition process.

The spin-coating deposition technique is one of the most used methods for thin film fab-

rication on flat substrates making use of the centrifugal force of the spinning substrate

to spread and uniformly coat its surface. The coating material is dissolved in a volatile

solvent that evaporates during the baking process. Attending to the viscosity, concen-

tration and rotation speed, it is possible to precisely control the thickness of the thin

films, with increasing angular speed leading to thinner films [55]. This process is widely

used for photoresist deposition on flat substrates for microfabrication through photo-

lithography processes. It is also used for metal oxide layers deposition using sol-gel

precursors [56]. Figure 2.2 represents the typical spin-coating deposition process. A

Laurell WS-650S-6NPP spinner (CEMUP-MNTEC) (Fig. 2.3) was used to deposit ZnO

thin films using a sol-gel solution.

FIGURE 2.3: (a) Laurell WS-650S spinner. (b) Precision hot plate for soft-baking.

2.1.3 Hydrothermal Growth Process

The hydrothermal process consists in heating a solution of zinc nitrate hexahydrate

[ZNH; Zn(NO3)2 · 6 H2O] and hexamethylenetetramine [HMTA; (CH2)6N4] until appre-

ciable hydrolysis (80 - 100 C) occurs, during a fixed period. To initiate such process,

a ZnO seed layer is first needed to obtain uniform growth of oriented NWs on top of

a substrate [48, 57]. Then, the sample is placed floating on the heated solution until

thermal degradation of the HMTA occurs, releasing hydroxyl ions that react with the


Zn2+ and form ZnO molecules [58]. The chemical reactions are summarized in the

following equations:

(CH2)6N4 + 6H2O←−→ 6HCHO+ 4NH3 (2.1a)

NH3 +H2O←−→ NH4+ +OH− (2.1b)

2OH− + Zn2+ −−→ ZnO(s) + H2O (2.1c)

In this project, the process was executed using a set-up similar to the one represented

in Fig. 2.4.

FIGURE 2.4: Representation of hydrothermal apparatus for ZnO NW growth on flatsubstrates.

2.1.4 Solochemical Method

Solochemical method is a type of low-temperature chemical reaction recently developed

for the production of zinc oxide nanopowders that involves the preparation of a zinc

complex solution and subsquent decomposition of the complex into ZnO [44]. This

chemical process can also be used for the production of other oxides such as Mn2O3

and NiO. Such process can also be named two-stage solochemical (TSSC) method

and is described by the following equation:

(NH4)2ZnO2 +H2O −−→ ZnO + 2NH4OH (2.2)

In this work, the method was performed using standard laboratory material starting

with the dropcasting of the zinc complex solution into the heated decomposing solution.

Then the mixture is dried, evaporating the solvent until obtaining a powder that is further

collected and characterized.


2.2 Characterization Techniques

In the following section we present the two main characterizing techniques used in this

project; scanning electron microscopy (subsection 2.2.1) and X-ray diffraction (subsec-

tion 2.2.2), the first one for morphological analysis and elementary identification and

the second for crystalographic analysis.

2.2.1 Scanning Electron Microscopy

Scanning electron microscopy (SEM) is a morphological characterization technique

which retrieves information from the sample’s surface. The interaction of the incid-

ent electron beam with the near surface atoms originates the emission of electrons and

photons like secondary electrons (SE), backscattered electrons (BSE) and X-rays. The

first ones (SE) result from ionization and reveal the topography of the sample’s surface,

while and the second ones (BSE), that result from elastic backscattering collisions,

show the atomic number relation, providing an image of the distribution of constituting

elements, where the brighter areas correspond to the elements with higher atomic num-

ber. As a complementary technique, the energy-dispersive X-ray spectroscopy (EDS)

analyzes the emitted X-rays and identifies the constituting elements of the scanned

area, generating EDS spectra allowing a qualitative and quantitative characterization.

All the SEM images were obtained with the high resolution FEI Quanta 400FEG scan-

ning electron microscope (CEMUP) retrieving images with a resolution down to 1.2 nm.

2.2.2 X-Ray Diffraction

FIGURE 2.5: X-ray beams interacting with crystal’s periodic lattice.


X-ray diffraction (XRD) is a common characterizing technique for determining the crys-

tallographic structure of a material. XRD collects and analyzes the scattered X-rays by

the sample after an incident X-ray beam interacts with the electrons in the atoms. The

diffracted waves interact between each other creating a diffraction pattern. If a group

of atoms are arranged in a periodic formation, they will yield a characteristic diffraction

pattern with distinct sharp interference maxima (peaks). The diffraction peaks follow a

condition given by Bragg’s Law:

2dhkl sin θ = nλ, (2.3)

where dhkl is the interplane distance with the Miller indices (hkl), θ is the angle between

the atomic plane and the incident beam, n is an integer indicative of the order of the

diffraction peak and λ is the wavelength of the X-ray beam. Figure 2.5 shows a 2D

representation of a periodic crystal with the incident and reflected X-rays indicating the

occurring interactions during a XRD scan.

In the case of a polycrystalline material, the XRD spectra allow us to estimate the

average crystallite size (DXRD) through the Williamson-Hall relationship [59]:

βtotal = βsize + βstrain =kλ

DXRDcosθ+ 4ηtanθ, (2.4)

where βtotal is the full width at half-maximum (FWHM) of the XRD peak, k the Scherrer

constant for spherical crystallographic grains (∼ 0.94), λ the incident X-ray wavelength

(0.154 18 nm), θ the diffraction angle and η the microstrain parameter.

XRD patterns were obtained at room temperature using a Siemens D5000 diffracto-

meter (IFIMUP-IN) in the locked-coupled mode (θ − 2θ) and with the Cu−Kα line with

a wavelength of 1.54 A. The most commonly used parameters were a scan range from

20 to 80, with 0.01 steps and a scan time of 8 s per step.

Chapter 3

Fabrication of ZnO Nanostructures

In this chapter we describe the fabrication and characterization of several zinc oxide

nanostructures, including nanoparticles (0D), nanowires (1D) and thin films (2D). The

first section explores the electrochemical deposition process, in which a potential dif-

ference is applied between a metallic contact (the anode), and a sample (the cathode),

both immersed in an electrolytic bath. The second section describes the chemical pre-

paration of a zinc oxide sol-gel and its spin-coating procedure. Such method allows the

production of ZnO thin films as ending samples or as an assisting layer for a hydro-

thermal process. The hydrothermal process is discussed in detail in the third section,

addressing subjects as the chemical solution, the growth procedure and variable para-

meters. The hydrothermal process is widely used for the growth of ZnO nanowires with

well defined characteristics. The last section describes the growth of ZnO nanoparticles

using a simple production method by chemical precipitation, typically dropcasting a zinc

complex into an heated solution.

3.1 Electrochemical deposition of ZnO thin films

The results were obtained following the technique already described in subsection 2.1.1

using polyethylene terephthalate (PET) with a transparent conducting indium tin oxide

(ITO) film as working electrode and an aqueous solution of zinc nitrate hexahydrate as

electrolyte.

21

Fabrication of ZnO Nanostrutures 22

ConcentrationTemperature 0.1 M 0.01 M

RT -1.0 V -1.1 V -1.5 V -1.0 V -1.1 V -1.5 V80 C -1.0 V -1.1 V -1.5 V -1.0 V -1.1 V -1.5 V

TABLE 3.1: Parameters tuned during the electrochemical deposition process: temper-ature (RT and 80 C), concentration of ZNH (0.1 and 0.01 M) and applied potential

(-1.0, -1.1 and -1.5 V vs Ag/AgCl).

3.1.1 Experimental procedure

To better understand the electrodepostion process of ZnO thin films, we first studied

the influence of three different parameters on the structure of the deposits: the concen-

tration of zinc nitrate hexahydrate (ZNH), 0.1 and 0.01 M; the electrolyte temperature,

room temperature (RT; ∼ 19 C) and 80 C, based on the literature [37]; and the applied

deposition potential, -1.0, -1.1 and−1.5V vs. Ag/AgCl. Table 3.1 summarizes the tuned

parameters.

To deposit ZnO thin films at room temperature we used the standard experimental

set-up, shown in Fig. 3.1(a), with the typical three electrode system: a Pt mesh as

counter electrode, a PET/ITO substrate as working electrode, and a Ag/AgCl reference

electrode. In order to deposit with the electrolyte heated at 80 C [Fig. 3.1(b)] a few

additions were made to the system: a hot plate that heats a water bath with the previous

set-up inside and an extra sealing lid on top to prevent great loss of electrolyte through

evaporation.

FIGURE 3.1: Schematic representation of the electrochemical deposition experimentalset-up at (a) room temperature and (b) 80 C.


3.1.2 Results and Discussion

The produced samples were fully characterized by scanning electron microscopy (SEM),

energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and the electrode-

position current transients and charge density curves were analyzed, enabling us to

establish proper relationships between the variable parameters and the deposited thin

films.

3.1.2.1 Deposition at room temperature

Six samples were electrodeposited at room temperature, using the parameters presen-

ted in Table 3.1, with standard substrate dimensions of 0.5 x 1.4 cm2 and for 1 hour

each.

FIGURE 3.2: SEM images of electrodeposited ZnO, at different scales, at room tem-perature for 0.1 M of ZNH with applied potentials of (a1 and a2) -1.0, (b1 and b2) -1.1

and (c1 and c2) -1.5 V.

Figure 3.2 shows the SEM images of the deposited samples using 0.1 M of ZNH, illus-

trating the different surface morphologies obtained when tuning the applied potential:

-1.0 V [(a1) and (a2)], -1.1 V [(b1) and (b2)] and -1.5 V [(c1) and (c2)]. All samples re-

vealed traces of Zn and O on the EDS spectra, with increasing relative counts of Zn

and O for lower potential, indicating an increase of the deposition rate at more negat-

ive applied potentials. The deposited films morphology reveal the nucleation of ZnO

nanostructures in the substrate surface for the highest (-1.0 V) potential, without the


formation of a continuous film. As shown in Figs. 3.2(a1), (b1) and (c1), the electrode-

position of ZnO tends to always form clusters of nanostructures without ever forming a

continuous film, which means that such deposition favors an island-like growth, creating

areas with higher ZnO density. In a later moment, the ZnO forms almost flat zones at

the top of the islands, as seen in Fig. 3.2(c1).

FIGURE 3.3: SEM images of electrodeposited ZnO, at different scales, at room tem-perature for 0.01 M of ZNH with applied potentials of (a1 and a2) -1.0, (b1 and b2) -1.1

and (c1 and c2) -1.5 V.

When changing the electrolyte concentration to a tenth of the previous amount, and

using the same above conditions, the prepared samples reveal a much different mor-

phology, displaying now planar microramifications of very small height, especially when

applying a potential of -1.0 or -1.1 V [Figs. 3.3(a) and (b)]. Furthermore, we can verify

that at -1.5 V the morphology of the deposited films has two stages: an earlier depos-

ition of microflowers [Fig. 3.3(c2)] followed by a top flat layer with nanospheres clustered

on the surface.

3.1.2.2 Deposition at 80 C

We further electrodeposited thin films with the electrolyte heated at 80 C, and keeping

the same standard parameters as in the previous samples (dimensions, electrodepos-

ition duration, electrolyte concentrations and applied potentials).

The obtained results for the electrodeposited samples in the 0.1 M electrolyte are shown

in Fig. 3.4 and can be compared with the ones deposited at room temperature (Fig. 3.2).


FIGURE 3.4: SEM images of electrodeposited ZnO, at different scales, at 80 C for 0.1M of ZNH with applied potentials of (a1 and a2) -1.0, (b1 and b2) -1.1 and (c1 and c2)

-1.5 V.

One immediately notices a clear change in the samples morphology upon decreasing

the applied potential to more negative values. At -1.0 V, ZnO nucleates in clustered

nanostructures, which are absent at lower deposition potentials. At -1.1 V, ZnO forms

localized webs that grow on top of each other, as seen in Fig. 3.4(b1) and (b2). On the

other hand, at -1.5 V, the formed nanostructures are much different. Thus the heating

of the electrolyte provided a change in the ZnO nanostructures compared to the room

temperature samples and to the different potentials applied at 80 C.

Finally, we electrodeposited ZnO using an heated electrolyte at 80 C, with a concen-

tration of 0.01 M. The produced samples (Fig. 3.5) revealed the nucleation of nano-

structures similar to those previously presented in Fig. 3.4(a), although with a lower

density, due to the lower concentration of the electrolyte. Differently from the 0.1 M

concentration, all the applied potentials show the same type of ZnO nanostructures,

with increasing dimensions for more negative potentials.

The crystallographic structure of the electrodeposited ZnO thin films was studied by

XRD. Figure 3.6 shows the XRD spectra of the substrate prior to the deposition process

and of a representative ZnO thin film sample (-1.1 V with a concentration of 0.01 M at

80 C). We can observe that the obtained spectra are very similar, thus showing only

the presence of the crystallographic peaks of the substrate. From such spectrum one

can say that the electrodeposition of ZnO does not produce crystalline structures with

defined crystalline planes but rather an amorphous (or nanocrystaline) phase.


FIGURE 3.5: SEM images of electrodeposited ZnO, at different scales, at 80 C for0.01 M of ZNH with applied potentials of (a1 and a2) -1.0, (b1 and b2) -1.1 and (c1 and

c2) -1.5 V.

20 30 40 50 60 70 80

PET substrate

I (u.

a.)

2 (º)

PET substrate + electrodeposited ZnO

FIGURE 3.6: XRD spectra of a PET substrate (red line) and a representative ZnOelectrodeposited thin film on top of a PET substrate (black line).

3.1.2.3 Deposition Current Transients

The monitorization of the current transients during the electrodeposition process allows

one to better understand the mechanism of ZnO formation. Figure 3.7 show the current

transients monitored during the electrodeposition process of selected ZnO thin films on

PET substrates with an ITO coating. From the obtained results we observe an increase


of the electric current with increasing decreasing potentia, at room temperature and

80 C [Fig. 3.7(a)]. Also, an increase in the electric current is verified when increasing

the solution concentration [Fig. 3.7(b)]. Finally, comparing representative electrodepos-

ited samples at RT and 80 C [Fig. 3.7(c)] we see that the electric current also increases

with the heated electrolyte. The process is temperature assisted, thus the heating of

the electrolyte increases the deposition rate.

Figure 3.7(d) shows the complete curve of the deposition process for a solution with

0.01 M of ZNH, at room temperature and under -1.5 V. We can define two character-

istic zones in the curve with different deposition behaviors. During the first 1250 s the

electric current drastically decreases until a minimum is reached, followed by a second

region where the electric current remains constant in a minimum value. Such behavi-

ors can be related with the results in Fig. 3.3(c1), where ZnO microflowers were initially

formed [Fig. 3.3(c2)], followed by a flat ZnO surface with nanospheres clustered to the

surface.

FIGURE 3.7: Deposition current transients recorded during the electrodeposition pro-cess of selected ZnO thin films electrodeposited at (a) room temperature and (b) 80 C.(c) Comparison plot of representative electrodeposited samples with varying electro-lyte temperature. (d) Complete i(t) current curve for a sample electrodeposited at RT

with 0.01 M of ZNH under -1.5 V.


3.1.2.4 Deposited Charge

Assuming that all the measured current is used in the electrodeposition of ZnO, i.e.

all flown current is used to deposit the Zn2+ ions and none is used in side chemical

reactions, we can then estimate the deposited charge through the integral calculus of

the deposition current curve over time (Q =∫idt). The chemical reactions taking place

at the substrate (PET with ITO) during ZnO electrodeposition are the following [60]:

Zn(NO3)2 −−→ Zn2+ + 2NO3− (3.1a)

NO3− +H2O+ 2 e− −−→ NO2

− + 2OH− (3.1b)

Zn2+ + 2OH− −−→ Zn(OH)2 (3.1c)

Zn(OH)2 −−→ ZnO + H2O. (3.1d)

Using the standard size of the PET/ITO substrates (0.5 x 1.4 cm2), we estimated the

deposited charge density for all prepared samples at room temperature and 80 C

[Fig. 3.8(a) and (b)]. We can verify that, for room temperature deposition, the charge

density increases rather linearly over time, during the first 500 s of deposition. The

charge density slopes increase with the ZNH concentration and with decreasing the

applied potential. Lowering the cathodic potential produces a faster increase in the

deposited charge than increasing the concentration. The same conclusions are ob-

tained for depositions at 80 C [Fig. 3.8(b)]. Q(t) also increases linearly during the first

500 s of deposition, showing a constant deposition rate without changes in the depos-

ition process. Figure 3.8(c) shows that the deposited charge also increases by heating

the electrolyte, leading to the maximum value for the larger negative potential (-1.5 V)

and at 80 C. This behavior may be explained by a phenomenon during the deposition,

where part of the deposited film peeled off, opening area for more deposition at the

substrate’s surface. This may occur because of the unstable electrodeposition when

applying lower potentials (-1.5 V), as already shown in Fig. 3.7. Also, we can verify that

the applied deposition potential produces larger variations of the charge density, with

the charge density at -1.5 V, 0.01 M and RT being higher than at -1 V, 0.1 M and at

80 C. Finally, we can analyze Fig. 3.8(d), where the complete charge density curve

of the sample electrodeposited for -1.5 V, 0.01 M of ZNH and at room temperature is

shown. This curve can be divided into two distinct regions, the first lasting for about

1000 s shows a linear behavior, similar to the other graphs, and a second with a much

lower slope (almost constant charge density) indicating a nearly null deposition. As in

Fig. 3.7(d), this curve can be related with the obtained SEM images [Figs. 3.3(c1) and

(c2)] where two distinct zones are also evidenced.


FIGURE 3.8: Time dependence of the electrodeposition charge density during thegrowth of representative ZnO thin films for comparison of the effect of increasing ZNHconcentration and applied potential at (a) room temperature and (b) 80 C. (c) Elec-trodepostion charge density plots comparing representative electrodeposited sampleswith varying electrolyte temperature. (d) Complete charge density curve over time for

the 0.01 M of ZNH sample under -1.5 V at room temperature.

Finally, the calculated deposited charge allowed us to estimate the thickness of the thin

film using Faraday’s laws of electrolysis equation [61, 62]:

m =QM

nF(3.2)

where m is the deposited mass in grams (g), M is the molar density of the deposited

material (81.40 gmol−1 for ZnO), Q is the deposited charge in coloumbs (C), n is the

eletronegativity of the deposited ions (n = 2 for Zn2+) and F is the Faraday constant (F

= 96 485Cmol−1).

Using the volumetric density of ZnO (ρ = 5.61 g cm−3) and the deposition area (A =

0.7 cm2) one can estimate the thin film’s thickness (h) through:

h =m

A.ρ(3.3)


The results are plotted in Fig. 3.9 where we can see that the highest thickness was

obtained at -1.5 V and 80 C for a concentration of 0.01 M, which is expected from what

we saw in the deposited charge curves [Fig. 3.8]. Furthermore, we can see that the

higher estimated thicknesses are obtained at 80 C which confirms the temperature as-

sisted behavior when raising the temperature of the deposition. At room temperature,

the highest thickness is obtained at -1.5 V, using the highest concentration (0.1 M), as

expected. At 80 C, the estimated thickness indicates a maximum h value for a concen-

tration of 0.01 M but, noticing the error bars in Fig. 3.9, the estimated thickness at -1.5

V and 80 C, for 0.01 M and 0.1 M are under the same error fluctuations, calculated

through the error propagation of Eq. (3.3) taking into account the measurement error

of the samples’ area, the estimated deposited mass and the ZnO density value, thus

disregarding the concentration influence in such case. Finally, we verify that the estim-

ated thickness tends to increase with the modulus of the applied potential. However, a

sudden increase is seen from -1.1 to -1.5 V, which may indicate an unstable deposition

at more negative potentials, as also evidenced by the distinct current transients and

inhomogeneous surfaces found at -1.5 V samples. In the estimated thickness of the

electrodeposited thin films we expected a linear increase with the decreasing applied

potential, but this sudden increase at -1.5 V can also be justified by the considerations

taken into account that the deposition efficiency was 100%. If the deposition efficiency

is lesser than 100%, the estimated thickness would be smaller. The lost in efficiency is

related with the formation of H2, thus consuming energy originally meant for the depos-

ition.

1,0 1,1 1,2 1,3 1,4 1,5

0

100

200

300

400

500

600

Hei

ght (

nm)

Potential (V)

0.1 M, RT

0.1 M, 80ºC

0.01 M, RT

0.01 M, 80ºC

FIGURE 3.9: Estimated thickness of the electrodeposited thin films.


3.1.3 Conclusions

A complete study was performed on the variations of the morphology of the ZnO thin

films on PET/ITO substrates with the ZNH concentration, applied potential and elec-

trolyte temperature. The increase of the temperature of the electrolyte and zinc nitrate

concentration, and the decrease of the applied potential were found to increase the

deposition current and thus the charge density. By varying the applied potential one

can greatly change the morphology of the electrodeposited films. For 0.01 M of ZNH

at 80 C, the nanostructures increase in size when decreasing the applied potential to

more negative values, maintaining the same shape. Also, the specific case of 0.01 M,

-1.5 V and 80 C shows two distinct morphologies with the deposition time, indicating

the influence of the applied potential during deposition.

Although this study led to important conclusions about the deposition of ZnO through

electrodeposition, it did not allow us to obtain homogeneous thin films appropriated to

apply in piezoelectric nanogenerators. We have thus undertaken the task to fabricate

ZnO thin films using the spin-coated sol-gel method.

3.2 ZnO thin film obtained by a sol-gel chemical process

In this section we produced ZnO thin films through a spin-coating deposition, as de-

scribed in subsection 2.1.2, of a ZnO sol-gel produced following the procedure ahead

presented.


A sol-gel solution was produced based on zinc acetate dihydrate (ZAD; Zn(CH3COO)2 ·2 H2O), monoethanolamine (MEA; C2H7NO) and anhydrous ethanol (EtOH) [63]. The

complexing agent, MEA, was dissolved in ethanol with a concentration of 1.2mol L−1

composing a solution of 100mL, stirred for 10min at 300 rpm. Furthermore, a zinc

complex solution with a molar ratio of 1:1.5 ([Zn2+] : [MEA]) was produced dissolving

the ZAD in EtOH and stirring it at 300 rpm for 10min. The later solution was then added

to the first one, heated at 60 C and stirred at 160 rpm with reflux for 2 h, as presented in

Fig. 3.10. We obtained a homogeneous, transparent and slightly viscous solution with

250mL. Afterwards, the solution was filtered with proper filtering paper and aged during

72 h in a glass container at room temperature.


ºC

FIGURE 3.10: Schematic representation of reflux process for ZnO sol-gel synthesis.

The depositions were performed on PET/ITO substrates, chosen for purposes explained

further ahead. The substrates were previously cleaned using a standard cleaning pro-

cedure in acetone, isopropanol, ethanol and di-ionized water, 5min in ultra-sounds,

each. Subsequently a proper amount of the sol-gel solution was dropped on the sub-

strate (over the face with the ITO thin film) covering all the area, and rotated at 4000

rpm for 30 s. The substrate was then placed over the hot plate at 150 C during 10min.

The process was finalized after six repetitions of the described procedure. The pro-

duced thin films can also act as ZnO seed layers for the hydrothermal process, which is

a required layer to favor the growth of ZnO nanowires, as will be detailed further in this

chapter. Produced samples were characterized by SEM and XRD analysis techniques.


The deposited thin films were morphologically analyzed by SEM top and cross-sectional

views to unveil characteristic aspects such as uniformity and topography. We found

a wavily surface distribution [Figs. 3.11(a) - (c)], with the ripples being hollow inside

[Fig. 3.11(d)]. This likely comes from successive compression and distension of the

deposited layer upon the heating phase (soft bake) at 150 C, thus creating a bended

ZnO film. The film thickness after the six repetitions was approximately 900 nm.


FIGURE 3.11: Morphological analysis of ZnO spin-coated sol-gel thin film: [(a) to (c)]top-view images of the spin-coated thin film at different scales; and (d) cross-section

view of the ZnO thin film.

The crystallographic structure of the ZnO films was characterized by XRD, as shown

in Fig. 3.12, revealing an amorphous (or nanocrystaline) spectrum similar to the ones

obtained for the electrodeposited ZnO nanostructures. Also in this case, we analyzed

the substrate before deposition and obtained similar XRD spectra which indicates that

the deposited ZnO is not crystalline nor with wurtzite structure implying that the used

sol-gel and spin-coating deposition does not favor the piezoelectric properties of the

zinc oxide.

3.2.3 Conclusions

The sol-gel route assisted by spin-coating allowed one to obtain ZnO thin films, with

great uniformity throughout a wide area. Although the films have an high roughness

and a wavily morphology, it is well deposited over the whole substrate. Other deposition

methods such as magnetron sputtering and ion beam deposition can prevail in thin film

quality over the spin coated sol-gel, but involving higher production costs, which leads

to their infeasibility on an industrial perspective. From a financial point of view, the

cost of sol-gel production involves a summed cost of approximately 8 to 10 e for six

spin-coated layers over around 30 x 30 cm2. On top of that we have to add the typical

cost of a primary vacuum bomb and typical consumables for the deposition process,

but overall is much more economic than a fully physical deposition. Furthermore, this


20 30 40 50 60 70 80

PET substrate + ZnO sol-gel thin film

I (u.

a.)

2 (º)

PET substrate

FIGURE 3.12: XRD spectra of the spin-coated thin film produced by a ZnO sol-gel (redline) and the used standard PET substrate (black line).

process allowed us to obtain the desired seed layer to favor the correct growth of ZnO

nanowires by a hydrothermal process (as described in the next section), thus producing

nanostructured nanogenerators.

3.3 Growth of ZnO nanowires though hydrothermal process

The hydrothermal method of growing 1D zinc oxide nanostructures (nanowires, nanor-

ods, nanoneedles or nanobelts) [48] was first reported by Andres-Verges et al. [46]

and although its discovery was important, the interest only grew when Vayssieres et

al. [47] accomplished controlled growth of ZnO nanowires on glass and Si substrates

through a hydrothermal method. Our results were obtained using the typical chemical

composites, ZNH and HMTA, and following the previously described method in subsec-

tion 2.1.3.


The reactant solutions in this work were produced using ZNH and hexamethylenetetra-

mine (HMTA) on an equimolar ratio (1:1) and dissolved in di-deionized water. To study

the growth of ZnO NWs through a hydrothermal process we devised an apparatus sim-

ilar to the one represented in Fig. 2.4 and studied several parameters influencing the

hydrothermal growth, such as reactant solution concentration, process time duration


and number of repetitions (1 or 2-step process). The heating temperature was kept

constant at 95 C, based on related literature. Table 3.2 shows the several parameters

tuned during the hydrothermal growth method.

# StepsDipping Time 1 2

2.5 h 25 mM 50 mM 25 mM 50 mM5 h 25 mM 50 mM 25 mM 50 mM

TABLE 3.2: Parameters tuned during the hydrothermal growth method: time (2.5 and5 h); number of steps (1 or 2); concentration of zinc nitrate (25 and 50 mM).


1-Step We started by analyzing the influence of the reactant solution concentration

and process duration on the hydrothermally grown ZnO NWs. Figure 3.13 shows SEM

images of the obtained NWs for a concentration of 25 mM during (a) 2.5 h and (b) 5 h.

Figure 3.14 shows the obtained results for a concentration of 50 mM during (a) 2.5 h

and (b) 5 h.

The obtained results denote the increasing density of NWs over the surface of the

substrate when increasing the concentration [Figs. 3.13 and 3.14] or the process dura-

tion [Figs. 3.13(a) and (b) or 3.14(a) and (b)]. For the established maximum conditions

[Figs. 3.14(b); 50 mM and 5 h] we obtain a highly dense distribution of NWs, with prefer-

ential vertical alignment, always normal to the surface morphology. In fact, the paramet-

ers variation also change the aspect ratio of the NWs, altering the height (H) and width

(W) of the produced nanowires. Table 3.3 shows the geometric parameters estimated

from the SEM images of the nanowires and the respective aspect ratio (H/W ).

Width (nm) Height (µm) Aspect Ratio

25 mM2.5 h 40 0.45 11.255 h 300 1.4 4.67

50 mM2.5 h 90 1.06 11.115 h 400 0.54 1.34

TABLE 3.3: Geometric parameters of hydrothermally grown ZnO NWs for differentsolution concentrations and process durations (1-step process).

Table 3.3 reveals an interesting behavior for the hydrothermal growth of ZnO nanowires.

The estimated values show that the width is relatively small (between around 40 and

90 nm) for the smallest time used, greatly increasing for a longer duration (5 hours),


where a width of 300 and 400 nm was estimated. This indicates that we can expect

even thinner NWs for quicker hydrothermal processes, not allowing the ZnO to grow

larger hexagons. As for the height, we could not find a monotonous trend, as for 25

mM an increase was seen with increasing dipping time but the opposite occurred for 50

mM. This behavior may be justified by the seed layer conditions and its adhesion to the

substrate which might affect the NWs height.

Although the height values are not entirely justified, we calculated the aspect ratio of the

nanowires in the different conditions and found that the aspect ratio tends to increase

for lower reactant solution concentration and smaller process duration, with the highest

value (11.25) occurring for the sample produced with a concentration of 25 mM and 2.5

h duration.

FIGURE 3.13: SEM images of hydrothermal samples with 1 step for 25 mM concen-tration during (a) 2.5 h and (b) 5 h.

2-Step For the 2-step growth of the ZnO nanowires, the previous procedure was re-

peated on new substrates, starting with the deposition of the necessary seed layer and

a first hydrothermal step following the above conditions. Then, the grown layers were

cleaned with deionized water and the hydrothermal process repeated under the same

conditions of the first step, aiming to evaluate the influence of a 2-step process in the

aspect ratio and distribution density of ZnO nanowires. Figures 3.15 and 3.16 show the

morphologic analysis obtained for 25 and 50 mM, respectively. First we point out the

lack of quality in some of the produced samples, [see Figs. 3.15(b)] where part of the

seed layer peeled off, not allowing the hydrothermal growth to occur in certain zones.

This led to a growth, during the first and second step, in an island-like deposition, as

shown in detail in Fig. 3.15(b3). Figure 3.16(a) also shows a defective growth of ZnO


FIGURE 3.14: SEM images of hydrothermal samples with 1 step for 50 mM concen-tration during (a) 2.5 h and (b) 5 h.

NWs, with zones without NW growth which indicates a peel of the seed layer. Although

some samples had these unexpected behaviors, we were still able to analyze the pro-

cess and trace interesting relations.

From the morphologic point of view, the SEM images reveal the formation of NWs, with

different degrees of vertical alignment and hexagonal shape. In normal conditions, we

are able to obtain a dense distribution of NWs, vertically aligned and covering all the

processed surface [Figs. 3.15(a) and 3.16(b)]. Furthermore, we were able to estim-

ate the geometric parameters and aspect ratio for each produced sample, which are

summarized in Table 3.4.

Width (nm) Height (µm) Aspect Ratio

25 mM2.5 h 230 1.4 6.095 h 700 2.4 3.43

50 mM2.5 h 600 1 1.675 h 400 2.05 5.13

TABLE 3.4: Geometric parameters of hydrothermally grown ZnO NWs for differentsolution concentrations and process durations (2-step process).

The estimated values show no visible tendency of the geometric parameters as function

of the production parameters (concentration and duration), which may indicate a wrong

estimation of measurements due to the defective samples.

In a comparative analysis between the 1-step and 2-step samples, we can denote that

overall, the height and width tend to increase with the number of steps, but not in a


FIGURE 3.15: SEM images of hydrothermal samples as produced using a 2-stepprocess for 25 mM concentration during (a) 2.5 h and (b) 5 h.

similar proportion, which leads to an overall fall in the aspect ratio. The maximum

registered aspect ratio occurs for the sample with the lowest variable parameters (in

both 1- and 2-step processes), due to the growth of nanowires with extremely small

width, which tends to increase the aspect ratio. Between the first and the second step,

the width tends to increase in the 2.5 h duration processes because, at the end of the

first step, the full growth of the NW as not yet been reached and the second step will

continue that growth. As for the 5 h processes, the width does not increase greatly

because the maximum value has already been reached in the first step.

FIGURE 3.16: SEM images of hydrothermal samples as produced using a 2-stepprocess for 50 mM concentration during (a) 2.5 h and (b) 5 h.


The XRD spectra of representative hydrothermally grown samples was also measured,

first in a Kapton tape substrate [Fig. 3.17; red line], and then on the PET/ITO substrate

(black line). The spectra reveal a distinct peak at 34.44 for both samples, attributed

to the (002) crystallographic plane (compare with the ZnO reference spectrum; blue

line) with similar intensities. Also, the PET sample spectrum shows a distinct peak

related with the substrate, as already shown in the spin-coated ZnO thin film analysis

(section 3.2). Furthermore, this sample also reveals three minor peaks, at 36.25, 47.53

and 62.87 that are attributed to the (101), (102) and (103) ZnO crystallographic planes,

respectively. We suggest that this happens because of a higher solution concentration

and longer process duration which leads to the growth of different crystalline planes.

Measuring the ratio between the intensity of the (101) peak in each spectrum (which

corresponds to the highest peak in the polycrystalline ZnO pattern XRD spectrum) and

the intensity of the peak associated with the crystalline plane responsible for the highest

value of the piezoelectric effect [(002) plane] we probe the influence of the ZnO growth

process on the desired crystalline direction. Using these intensity values, the ratio

between the planes (002) and (101) for the reference spectrum is approximately 0.46,

where for the hydrothermal grown 50 mM, 5 h spectrum is approximately 5.76. This

means that the hydrothermal growth allows the fabrication of enhanced piezoelectric

ZnO nanowires as they exhibit a highly preferential growth along the (002) direction.

30 40 50 60 70 80

I (u

.a.)

2 (º)

ZnO Spectrum 25 mM, 2.5 h (Kapton) 50 mM, 5h (PET)

(100)(002)

(101)

(102)(110) (103)

(200)

(112)(201)

(004)(202)

(PET/ITO peak)

FIGURE 3.17: XRD spectra of hydrothermally grown ZnO nanowires.


3.3.3 Conclusions

We studied the growth of ZnO NWs fabricated through hydrothermal processes varying

the zinc nitrate concentration, process duration and number of steps. The obtained

results showed the ability to control the geometric parameters, height and width, of

the ZnO nanowires and the respective crystallographic structure. For the lowest con-

centration value and process duration, we obtain the highest aspect ratios which can

improve the mechanical to electrical conversion rate, a parameter of great importance

for a piezoelectric nanogenerator, as will be shown in chapter 4.

3.4 Fabrication of ZnO nanoparticles by a solochemical method

The fabrication of ZnO nanoparticles followed the solochemical method, previously de-

scribed in subsection 2.1.4 using zinc nitrate hexahydrate (ZNH) as the zinc complex

solution and sodium hydroxide (NaOH) as the decomposing heated solution. The ex-

perimental procedure is subsequently detailed and followed by the obtained results and

drawn conclusions.


We performed a simple solochemical procedure in order to create ZnO nanoparticles

[45], starting from a solution of 0.5 M of zinc nitrate hexahydrated (ZNH) in 40mL of

di-deionized water. A reactant solution of 1 M of sodium hydroxide (NaOH) in 40mL

was heated up to 70 C and stirred at 150 rpm. Then, the ZNH solution was dropcasted

into the heated NaOH using the experimental procedure presented in Figure 3.18(a),

attaching a burette to a support with a claw and positioning the dropping rate screw

to a rate of approximately 1 drop per second. After 1 h of dropcasting, the burette

was removed and the resulting mixture was kept at 70 C and 150 rpm for 2 h allowing

the evaporation of the exceeding solvent [water; Fig. 3.18(b)]. Afterwards, the stirring

was stopped and the container was sealed and kept at 70 C for another 30min [Fig-

ure 3.18(c)]. Finally, we performed a soft dry of the resulting precipitation by opening

the container and keeping the temperature between 60 to 100C for 1 h, as shown in

Fig. 3.18(d). The resulting powder was then collected and stored in eppendorfs. Cal-

cinations on separate samples were performed at 500 and 800 C in air. All samples

were characterized by SEM and XRD.


FIGURE 3.18: Peparation steps of ZnO nanoparticles: (a) zinc complex solution drop-casted into heated NaOH solution for 1 h; (b) evaporation of exceeding solvent for 2 h;(c) removal of the remanescent water with sealed beaker for another 30min; (d) soft

dry of the ZnO powder.


FIGURE 3.19: SEM images of (a) clusters of ZnO nanoparticles, and (b) ZnO nano-particles with hexagonal shapes without an annealing step.

The morphological analysis of the ZnO nanopowder, performed by SEM (Figure 3.19),

revealed 20 µm width clusters composed of ZnO nanoparticles between 0.2 and 1 µm of

width and nearly hexagonal shapes. In fact, as shown in Fig. 3.19(b), the obtained ZnO

nanoparticles are not spherical shaped, but rather hexagonally shaped mostly due to

the ZnO wurtzite phase that leads to hexagonal nanostructures and the piezoelectric

properties of the ZnO. Furthermore, a crystallographic analysis was performed to fully

characterize the obtained samples. Figure 3.20(a) shows the XRD spectrum of the


ZnO nanopowder without any annealing step, revealing the presence of ZnO with the

typical crystallographic peaks of the ZnO wurtzite structure. The crystallographic ana-

lysis of the samples annealed at 500 C and 800 C [Fig. 3.20(a)] was also performed

to determine the effect produced by a thermal cooking of the powder in the crystalline

structure of the nanoparticles. The XRD spectra were also compared, estimating the

ratios between the intensities of the (101) and (002) peaks. For the spectrum without

annealing one calculated a ratio of 0.54, slightly higher than the obtained for the ZnO

reference spectrum in Fig. 3.17. For the samples annealed at 500 C and 800 C a ratio

of 0.46 was obtained, which is the same as for the ZnO reference spectrum. Although a

small crystallographic texture along the <002>direction is found for the sample without

annealing, the estimated ratios are very close which indicates that the annealing step

causes little effect in the crystalline structure of the nanoparticles.

The average crystallite size of the particles (DXRD) was estimated using the Williamson-

Hall relationship shown in Eq. (2.4). Figure 3.20(b) shows the estimation of DXRD, with

the respective values indicated, for each sample. Higher annealing temperatures lead

to an increase of the average particle size. This trend likely indicates an higher crystal-

lization of the nanoparticles, forming larger structures, with more regular shapes. For

the annealing at 500 C the estimated crystallite size decreased compared to the one

without annealing, which might have happened due to bad estimations in the FWHM of

the spectrum peaks.

FIGURE 3.20: (a) XRD spectra for the resulting ZnO nanopowder: without annealingstep (black line), after an annealin step at 500 C (red line) and at 800 C (blue line). (b)

Linear fit using the Williamson-Hall correlation.

3.4.3 Conclusions

ZnO nanoparticles were produced based on the solochemical method and further mor-

phologically and crystallographically characterized by SEM and XRD, revealing particles

with a diameter between 200 nm and 1 µm with a polycrystalline ZnO wurtzite structure.


Annealings at different temperatures (500 C and 800 C) were performed but no effect

was found on the crystallographic structure, thus maintaining the same piezoelectric

properties of the fabricated ZnO nanoparticles. Also, through the Williamson-Hall re-

lation we estimated the average crystallite size of the nanoparticles, concluding that

increasing the annealing temperature leads to larger crystallites.

3.5 Chapter Overview

Throughout this chapter we produced different ZnO nanostructures with the intent to

apply them in the production of a piezoelectric nanogenerator. Starting with the elec-

trodeposited thin films, although they were almost uniformly deposited in the substrate,

we could not obtain the (002) crystallographic peak necessary for the piezoelectric ef-

fect. Subsequently we projected a piezoelectric NG prototype composed by ZnO nan-

oparticles and through the solochemical method we produced them, but the prototype

production failed due to lack of the dropcasting set-up and the nanoparticles dispersion

solution. Also, the nanoparticles XRD spectra did not show the desired preferential

crystalline direction <002>. We then followed to the growth of ZnO nanowires, starting

with the deposition of a necessary ZnO seed layer, deposited by spin-coating a sol-

gel. The ZnO nanowires grown byhydrothermal process allowed us to produce our first

functional piezoelectric nanogenerator as further described in chapter 5.

Chapter 4

Finite Element Methods Study

In this chapter we study, through a finite elements method (FEM), the piezoelectric

output potential and deformation behavior of a 3D hexagonal zinc oxide nanowire, ex-

ploring the output piezoelectric potential when applying a force parallel or perpendicular

to the top surface. Furthermore, several parametric sweeps are performed, evaluating

the behavior of the output potential when varying the height or width of the NW.

4.1 Hexagonal ZnO Nanowire

Zinc oxide nanowires are the fundamental piezoelectric nanostructures that compose

a piezoelectric NG so that the study of their behavior is crucial to optimize the working

conditions and potentialities of such components.

4.1.1 Numerical Methods

A hexagonal nanowire [Fig. 4.2(a)], defined by its side width (W ), and height (H), was

designed and all simulations were made in the stationary mode. The piezoelectric

system was defined by the mechanical equilibrium condition when there is no body

force f(b)e = 0 acting on the NW [64]:

∇.σ = f (b)e = 0, (4.1)

where σ is the stress tensor, which is related to the strain ε, electric field E and electric

displacement D, by the constitutive equations:

44

Finite Element Methods Study 45

σµ = cλµεµ − eiµEi

Di = eiµεµ + ϵijEj

, (4.2)

where cλµ is the Young’s modulus, eiµ is the stress piezoelectric coefficient and ϵij is the

permittivity constant, with i, j = 1, 2, 3 and λ, µ = 1, 2, 3 . . . 6. For an arbitrary applied

force direction, eiµ for the ZnO is given by the following matrix, accordingly to Voigt’s

notation:

eiµ =

0 0 0 0 e15 0

0 0 0 e15 0 0

e31 e31 e33 0 0 0

(4.3)

Having, as already referred, e33 =1.57Cm−2, e31 =−0.36Cm−2 and e15 = −0.36Cm−2

[65].

Finally, assuming no free charges, ρ(b)e = 0 , one has:

∇.D = ρ(b)e = 0. (4.4)

Furthermore, several boundary conditions were defined:

i The boundary load, i.e. the face where the force is applied, was defined as the

top face of the NW. Accordingly, one has σ.n = FA, with FA = F totA , where n is the

directional vector of the applied force per area, FA, defined as the total applied force,

Ftot, divided by the area (A).

ii The fixed constraint u = 0, that models the structurally blocked face of the system,

is here defined as the bottom face of the NW.

iii The ground, or zero electric potential plane (V = 0), was also defined as the bottom

face.

Our study was then divided in four sets of simulations to construct a complete diagnostic

of the electric potential. This was estimated as a function of an external force (F ),

varying from 0 to 1µN, in 0.2 µN steps, applied parallel [Fig. 4.2(b)] and perpendicular

[Fig. 4.2(c)] to the boundary load surface of a NW whose height and width were varied

from 0.5 to 3µm (in steps of 0.25 µm) and 50 to 500 nm (in steps of 50 nm), respectively.

The four sets of simulations considered the variation of the force in a NW with constant

height (H = 3µm) and width (W = 100 nm), as shown in Fig. 4.2(d); the variation of the


force and height in a NW with a constant width (W = 100 nm), as shown in Fig. 4.3;

the variation of the force and width in a NW with a constant height (H = 3µm, Fig. 4.4);

and the variation of the height and width of a NW, when applying a constant force of

1µN (Fig. 4.5). All simulations were performed using the finite element method, with

the applied force along both the parallel and perpendicular directions.

4.1.1.1 Mesh details and limiting information

FIGURE 4.1: Mesh definition for nanowire geometry showing (a) the mesh distributionover the 3D nanowire, (b) in a close-up perspective and (c) over the top surface.

The delimiting mesh of the geometry (Fig. 4.1), a tetrahedral fine division using the

simulation software COMSOL R⃝, comprehending a maximum element size between 30

and 240 nm, with a maximum element growth rate of 1.45 and a resolution of narrow

regions of 0.6. This allows us to compute the system with an average precision in

values and in useful time, without the need of large computing resources.


Figure 4.2(d) shows the behavior of the electric potential as a function of the applied

force, depending on the direction, for a NW with H = 3µm and W = 100 nm. As dis-

played, the electric potential increases much faster for a perpendicular compressive

force (F z) than for a parallel bending force (F x). Furthermore, the maximum force of

1µN resulted in an output of 0.7 V for F x and 18 V for F z. Assuming a single crystal

of ZnO and according to Voigt’s notation [Eq.(4.3)] , the piezoelectric coefficients are

higher for a materials displacement along the z-axis (e33) than for a displacement along

the x-axis (e15), which results in a lower electric potential along the parallel direction for


FIGURE 4.2: (a) Considered geometry (height H, and width W) of a zinc oxidenanowire. Perspective view of a deformed NW upon (b) parallel and (c) perpendic-ular applied forces (F). (d) Electric potential as a function of applied force for both

parallel and perpendicular directions.

the same force. Analyzing Fig. 4.2(d), we extracted two simple equations to describe

the electric potential as a function of the strength and direction of the applied force:

V Fx = 0.7F , (4.5)

and

V Fz = 18F . (4.6)

Therefore, we were able to determine the behavior of the electric potential for a com-

pressed and a bended nanowire. The next step is to understand the influence of the

geometric parameters on the electric potential, when such deformations are applied.

For the following analyses, upon a parallel applied force, we have limited the maximum

bending angle to 30 due to physical constrains of the NW [66, 67]. Thus, the maximum

deformation of the top face of the nanowire was limited to:

L =2πH

360θ (4.7)

with L as the arc length and θ as the maximum NW bending angle (here 30, further

limiting the calculated electric potential).

Figures 4.3(a) and (b) show the electric potential behavior of a ZnO NW with W =

100 nm, upon applying a parallel bending force F x [Fig. 4.3(a)] and a perpendicular

compressing force F z [Fig. 4.3(b)], as a function of the strength of the applied force

and the NW height. In Fig. 4.3(a) one can see that, when a constant force is applied,

the output potential is approximately constant for the various NW heights. This implies


FIGURE 4.3: Electric potential as function of the height of the NW and the (a) paralleland (b) perpendicular applied forces, considering W = 100 nm.

an almost independence between the electric potential and the height of the NW, as

described in Eq.(4.8). However, the highest electric potential obtained is approximately

1.175V for F x = 1 µN and H = 1.75 µm. Figure 4.3(b) shows a much different beha-

vior, where considerably higher values of electric potential can be obtained, reaching a

maximum value of 18V when considering a NW with H = 3 µm and an applied perpen-

dicular force of 1µN, which deforms the NW by 0.03%. In addition, the output voltage

was found to linearly increase with the NW height for a perpendicular applied force, as

shown in Eq.(4.9). From these results we conclude that, depending on the applied force

direction, the influence of the height of the NWs may be of importance. Furthermore,

considering the maximum applied force of 1µN, we can determine the characteristic

behavior of the electric potential as a function of the NW height, as follows:

V Fx = 0.09H + 0.98 , (4.8)

and

V Fz = 6.0H − 0.03 . (4.9)

Figure 4.4 displays the electric potential behavior as a function of the applied force

and width of a NW with 3 µm in height. For a parallel applied force [Fig. 4.4(a)], the

output potential increases exponentially with decreasing NW width [Eq. (4.10)]. Such

behavior is also seen in Fig. 4.4(b), for a perpendicular applied force, although with

much higher values of electric potential [Eq. (4.10)]. The maximum electric potential

values obtained are 0.8V for F x = 1 µN and W = 150 nm, and 72.2V for F z = 1µN and

W = 50 nm, confirming again the higher output potential for a compressive force. From

such results one was also able to extract the behavior of the electric potential as a

function of the NW width, for a constant applied force of 1µN:


V Fx = 0.3 + 4.5e−0.015W , (4.10)

and

V Fz = 1.8 + 282.3e−0.028W . (4.11)

Analyzing both equations we can see that the output potential for a perpendicular force

decreases faster than for a parallel force for increasing values of width, where the

absolute value of the exponential constant is higher for V Fx (0.028 nm−1) than forV Fz

(0.015 nm−1), but reaches much higher voltage values for the first (282.3 V) than for the

later (4.5 V), as displayed in Figure 4.4.

FIGURE 4.4: Output voltage obtained by varying the width of the NW, while applying a(a) parallel and (b) perpendicular force, considering H = 3 µm.

Finally, we analyzed the electric potential obtained as function of the geometric NW

parameters, height and width, under a constant force of 1 µN applied along both parallel

[Fig. 4.5(a)] and perpendicular [Fig. 4.5(b)] directions. The results plotted in Fig. 4.5(a)

allowed us to confirm that, when applying a parallel bending force, the electric potential

is independent on the NW height (for the ranges studied) and increases exponentially

with decreasing width. On the other hand, when applying a constant force along the

perpendicular compressive direction, the electric potential linearly increases with the

NW height and exponentially increases with decreasing width [Fig. 4.5(b)]. The max-

imum values of electric potential obtained were again 1.2 V for F x = 1 µN on a NW with

H = 1.75µm and W = 100 nm, and 72.2 V for F z = 1 µN on a NW with H = 3µm and W

= 50 nm.

4.1.3 Conclusions

Numerical simulations of the deformation of a piezoelectric zinc oxide nanowire when a

force is applied, allowed us to extract the influence of the geometric parameters on the


FIGURE 4.5: Resulting piezoelectric potential upon a constant (a) parallel and (b)perpendicular applied force, as a function of the NW geometric parameters, height

and width.

output voltage, depending on the force strength and direction. For a parallel bending

force, our simulations showed that only the width of the NW influences the electric

potential, which increases exponentially with decreasing width, reaching a maximum of

1.2 V for 1µN of applied force. In the case of a perpendicular compressive force, the

output voltage is much higher, reaching up to 72.2 V when a force of 1 µN is applied. In

this case, the electric potential was found to increase linearly with the NW height and

exponentially with decreasing width. The output voltage obtained when compressing a

ZnO NW, approximately sixty times higher than when applying a bending force, can be

assigned to the much bigger piezoelectric coefficients of the wurtzite ZnO structure for

displacements along the z direction. Such results provide us crucial information and a

clear path for the future optimization of ZnO nanowires growth processes, envisioning

the fabrication of customized piezoelectric nanogenerators with high efficiencies.

Chapter 5

Piezoelectric Nanogenerator

In this chapter we explore the possibilities of producing a piezoelectric nanogenerator

based on zinc oxide nanowires. Through the following sections we will describe the

typical structure and composition of a piezoelectric nanogenerator and the production

methods used for its assembly. Furthermore, we developed a homemade test appar-

atus to measure the output potential obtained from the produced devices with standard

dimensions as a function of the applied bending force and frequency. Finally, we detail

some of the possible applications of such devices in textiles or shoes.

5.1 Composition and production

A typical ZnO based piezoelectric nanogenerator is composed of zinc oxide nanostruc-

tures between two metallic electrodes, assuring the electrical path to an external circuit,

as shown in Fig. 5.1(a). However, as we verified in preliminary tests, one cannot read

the output potential of a NG only with two electric contacts separated by the ZnO, be-

cause the resulting voltage upon deformation gets muzzled by the electric noise due

to the low electrical resistance of the device (approximately 100 kΩ). Such impediment

leads to the necessity of introducing an extra insulating layer to further separate the

two electric contacts, as detailed in Fig. 5.1(b) by the transparent layer surrounding

the ZnO hexagonal nanowires. This NG composition shows an electric resistance of

approximately 10MΩ.

The piezoelectric nanogenerators developed in this study were produced on PET sub-

strates with a coating of ITO and followed the methodical process of deposition and

chemical growth detailed in sections 3.2 and ??. Figure 5.2 summarizes the several

fabrication steps used in this work for the development of a piezoelectric nanogenerator.

51

Piezoelectric Nanogenerator 52

(a) (b)

FIGURE 5.1: (a) Graphical representation of a complete piezoelectric nanogenerator.(b) Sectioned nanogenerator displaying involved components.

The production starts with the deposition of a zinc oxide seed layer [step 1; Fig. 5.2(a)]

by spin-coating a ZnO sol-gel solution using 4000 rpm for 30 s followed by a soft-baking

at 150 C for 10min. This process was repeated six times, leading to a 900 nm thick

layer of ZnO over the ITO with exception of a small zone protected by Kapton (bottom

electrode), for electric connection purposes. Subsequently, ZnO nanowires were grown

on top of the seed layer through the previously described hydrothermal process [step

2; Fig. 5.2(b)]. Furthermore, the ZnO nanowires are mechanically coupled and elec-

trically isolated [step 3; Fig. 5.2(c)] by a spin-coated layer of S1818 photoresist with

approximately 2 µm in thickness using 3500 rpm during 30 s followed by a soft-baking

at 110 C during 2min. The final step [step 4; 5.2(d)] was the thermal evaporation of

a thick Al film over the S1818 layer, creating the upper electrode. Figure 5.1(b) shows

in detail the composition of the produced piezoelectric nanogenerators, identifying the

bottom contact, ZnO seed layer, ZnO nanowires, photoresist insulating layer and upper

contact.

Preliminary tests were performed on a representative device produced by the described

process. The device was placed inside a small metal box with a Bayonet Neill-Concelman

(BNC) connector [Fig. 5.3(a)] and connected to it using thin copper wires. The metal

box acted as a Faraday cage, shielding the obtained signal from electromagnetic noise.

The deformation was made by compreesing the NG using a pressuring device through

a hole in the box’s lid. The signal was detected by a Tektronix TDS 2024 oscilloscope

(IFIMUP-IN) where voltage peaks of approximately 50mV were recorded, as shown in

Fig. 5.3(b).


FIGURE 5.2: Steps of production of a piezoelectric nanogenerator: (a) deposition of aZnO seed layer; (b) hydrothermal growth of ZnO hexagonal nanowires; (c) spin-coating

of S1818 photoresist; and (d) thermal evaporaton of the Al upper contact.

5.2 Automatized set-up for mesurement of piezoelectric out-

put potential

After the production of the first functional piezoelectric NGs we created a systematic

testing method capable of evaluating the output potential from a nanogenerator upon

its bending with a constant controlled force and frequency of deformation.

5.2.1 Experimental Setup

Figure 5.4 shows the experimental set-up used for the deformation tests, where the

nanogenerator and respective electric contacts are holded to the set-up by two clamps.

One of the clamps is fixed, and the other is attached to an arm that moves with a con-

necting rod system rotated by a Parallax Inc. High Speed Continuous Rotation servo,

sliding forth and back the sample. The servomotor is controlled using a Parallax Inc.

BASIC Stamp R⃝ HomeWork BoardTM

microcontrolled module, fed by a homemade ten-

sion source at 6 V that is connected to the computer and handled using a LabVIEW


(a) (b)

FIGURE 5.3: (a) Metal box acting as Faraday cage, shielding the device from E.M.noise with NG connected with cooper wires. (b) Output potential obtained the from

first functional piezoelectric nanogenerator.

Nanogenerator

Oscilloscope

Microcontroller module

Metallic rail

High speed servo

Connection rod

FIGURE 5.4: Deformation output potential mesurement set-up.

program already developed by Bernardo Bordalo. This set-up allows one to test nano-

generators with different dimensions.

Unfortunately, due to time constrains, we were not able to prepare and test new piezo-

electric nanogenerators using the developed set-up. Nevertheless, we still tested the

working concept and verified that this set-up allows us to proceed with the development

of this technology in future work.


5.3 Applications

This work allowed us to envision several potential applications for the developed piezo-

electric nanogenerator, specially in the textile, shoes and technology industries. For

the first one, by placing a NG into a textile chest piece, such as a t-shirt, particularly in

the most deformed parts like the borders, one can actively produce electricity to store

in external circuits and further feed pocket electronics. These NGs can also be used

to permanently feed luminous electronics embedded in the cloth piece, such as LEDs,

which can then help to signalize the location of the consumer in dark places. In the

shoes industry, the piezo-NGs can be applied to the sole of the shoe, as already done

by certain companies using similar devices, but now allowing one to gather far more

energy due to the nanostructured devices, which are optimized to obtain the maximum

ouput voltage in the least useful area. In addition, the converted energy can also come

from the shoes bending or compressive pressure, thus enjoying from all mechanical

energy. Finally, we also envisioned a piezoelectric nanogenerator inside an electronic

gadget of touch nature, such as a mobile phone, therefore converting every stroke on

the touch surface into electricity, which can feed the battery of the device and increase

its autonomy.

Chapter 6

Conclusions and Future Work

The piezoelectric nanogenerator technology is a fairly recent development with no more

than a decade of existence. We can now envision a technological future with less ener-

getic waste, more energy harvesting and a more independent life using such nanogen-

erator devices. In a near future we will be able to place these nanogenerators in every

wearable technology, electronic gadgets, clothes and shoes, being able to feed small

electronics and sensors that will improve our quality of life. We can also envision the

application of this technology in the medical industry, enjoying the human movement to

feed diagnosis equipment or life support technology. Finally, we can find great use in

the environment monitoring of forests, rivers, deserts or even specific ecosystems, us-

ing sensors permanently fed by durable nanogenerators that convert the environmental

mechanical energy into electricity.

In this project we aimed to the prototyping and development of a piezoelectric nano-

generator based on ZnO hexagonal nanowires through a simple and low cost process,

with the possibility to scale-up to industrial dimensions. We chose ZnO as piezoelectric

material due to its overall properties and growth possibilities, as described in chapter 1.

In chapter 3 we presented four different ZnO nanostructures, grown by the techniques

described in chapter 2, to implement in the nanogenerator: electrodeposition, spin-

coated sol-gel solution, hydrothermal process and solochemical method. From those,

only the hydrothermal process allowed us to create ZnO hexagonal nanowires with a

preferential (002) crystalline direction attributed to the piezoelectric effect and allowing

us to control the geometric parameters (height and width) of the nanowires. Thus, this

was the method chosen to produce the piezoelectric nanogenerators in this project.

chapter 4 shows a numerical study on the influence of the geometric parameters of a

ZnO hexagonal nanowires on the output piezoelectric potential, thus showing us the

best parameters to apply in the hydrothermal process for our nanogenerator, depend-

ing on the deformation force. The study revealed that the highest output potential is

56

Conclusions and Future Work 57

obtained for the highest aspect ratio (biggest height and smallest width). Finally, in

chapter 5 we presented our path to produce a piezoelectric nanogenerator using low

cost chemical methods such as a spin-coated sol-gel solution and hydrothermal pro-

cess over PET substrates with a transparent conducting oxide. In this chapter we also

show some of the output potentials obtained during preliminary deformation tests using

metal box with the NG inside, connected to an oscilloscope. Futher, we developed a

testing set-up but were not able to test new nanogenerators.

As future work, we want to start exploring the effects produced by changing the condi-

tions of the hydrothermal process in the output potential for standard nanogenerator di-

mensions and applied deformation, thus defining the best production conditions. Then,

we want to create a reliable production process of NGs and test them thoroughly using

the testing set-up. Furthermore, we aim to apply this technology in low-energy con-

sumption electronics and test the applications in real-life situations, defining the actual

limits of our nanogenerators.

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